Task/Issue URL: https://app.asana.com/1/137249556945/project/72649045549333/task/1214798984829406
Tech Design URL: N/A
CC: N/A

### Description

- Adds `SubscriptionPromoExistingUserCoordinator` — a new launch-prompt coordinator targeting existing users (7+ days since install) who completed linear onboarding normally and have never seen a subscription offer. Shows a half-sheet with a CTA to the purchase flow using the `funnel_modal_ios__subscriptionnudge` funnel origin.
- Adds `subscriptionPromoForExistingUsers` remote feature flag (backed by `PrivacyProSubfeature.subscriptionPromoForExistingUsers`), guarded additionally by the existing `privacyProOnboardingPromotion` gate.
- Adds `SubscriptionFunnelOrigin.existingUserPromo` (`funnel_modal_ios__subscriptionnudge`) and three pixel events: `m_privacy-pro_existing_user_promotion_impression`, `…_tap`, `…_dismiss`.
- Refactors the per-provider onboarding gate: removes the global `hasSeenOnboarding` guard from `ModalPromptCoordinationService` and replaces it with a per-provider `isEligibleToPresent(isOnboardingComplete: Bool) -> Bool` method on `ModalPromptProvider`. The manager resolves the onboarding state once and passes it to each provider, keeping a single source of truth.
- `SubscriptionPromoExistingUserCoordinator` implements a softer gate: eligible when onboarding is complete **or** no contextual onboarding dialog is currently visible on screen (`isShowingContextualOnboardingDialog`) — allowing single-tab users who never reached the fire/EOJ flow to see the launch promo, while preventing the promo from appearing on top of active onboarding dialogs on cold launch.
- All other providers (`WinBackOffer`, `DefaultBrowser`, `NewAddressBarPicker`, `WhatsNew`) use the default implementation, which returns the passed `isOnboardingComplete` value — preserving their previous behaviour without each owning an `onboardingStatusProvider` dependency.
- Inserts `subscriptionPromoExistingUser` at priority 3 in the modal prompt queue (after reinstaller promo, before address bar picker).
- Adds `SubscriptionPromoUITests` covering promo appearance after onboarding and suppression while contextual onboarding dialogs are active.

### Testing Steps

**Prerequisites**
- Build from `alex/subscription-day7-promo`.
- Be an Internal User (add `|| DEBUG` to the `#if` check in `InternalUserStore.swift`).
- The cooldown is currently overridden to **1 minute** from install date for easier manual testing (production will require 7 days).
- **"Stop app"** = force-quit via Xcode Stop button or app switcher swipe-up.
- **"Hide app"** = press home button / swipe up to background without quitting.
- **Day-7 promo sheet** = the "Did you know?" half-sheet (title: *"Did you know?"*, CTA: *"Try it free!"* or *"Learn More"*, dismiss: *"No thanks"*, close: ✕ icon top-right).

---

#### A — Linear onboarding "Search only" path

To reach this state: complete linear onboarding → on the *"Want easy access to private AI chat in the address bar?"* screen select **"Search only"** → tap **"Next"** → complete remaining steps.

Contextual onboarding then starts on the browser. The *"Ready to get started? Try a search!"* dialog appears with search suggestion chips.

1. **"Try a Search" dialog visible; hide app, restore; stop app, cold launch — no promo both times**  
   On the *"Ready to get started? Try a search!"* dialog, do not tap any chip → press home button → switch back to DuckDuckGo → **No promo sheet** (foregrounding) → force-quit → wait ≥1 min → cold launch  
   → **No promo sheet**

2. **Select a search chip; "Try Visiting a Site" appears; hide app, restore; stop app, cold launch**  
   On *"Ready to get started? Try a search!"* select a search suggestion chip → SERP loads → *"That's DuckDuckGo Search!"* dialog → tap **"Got it!"** → *"Next, try visiting a site!"* appears → press home button → switch back to DuckDuckGo → **No promo sheet** (foregrounding; `isShowingContextualOnboardingDialog = true`) → force-quit → wait ≥1 min → cold launch  
   → **Day-7 promo sheet appears** (`tryVisitASiteShown` already persisted; dialog won't reappear; `isShowingContextualOnboardingDialog = false` on fresh launch)

3. **"Try Visiting a Site" dialog visible; tap X; stop app; cold launch**  
   On *"Next, try visiting a site!"* tap the **X** (skip/close) → confirm skip if prompted → force-quit → wait ≥1 min → cold launch  
   → **Day-7 promo sheet appears** (fire tutorial not started; `subscriptionPromotionPending = false`)

4. **Complete "Try Visiting a Site"; fire button tutorial shows; stop app; cold launch**  
   Visit a site from the *"Try Visiting a Site"* step → tracker dialog → **"Got it!"** → fire button is highlighted by tutorial → force-quit without tapping fire → wait ≥1 min → cold launch  
   → **No promo sheet**

5. **Press 🔥 fire button; contextual subscription promo "Oh, before I forget..." appears; hide app, restore; stop app, cold launch — no promo both times**  
   Follow fire tutorial → tap the 🔥 **Fire button** → fire confirmation → contextual subscription promo *"Oh, before I forget..."* appears → press home button → switch back to DuckDuckGo → **No promo sheet** (foregrounding; `subscriptionPromotionPending = true`) → force-quit → wait ≥1 min → cold launch  
   → **No promo sheet** (`subscriptionPromotionPending = true`; launch promo blocked to avoid clash with contextual flow)

6. **Tap "Try it free!" / "Learn More" on "Oh, before I forget..."; stop app; cold launch**  
   On the *"Oh, before I forget..."* contextual promo, tap **"Try it free!"** (or **"Learn More"**) → force-quit → wait ≥1 min → cold launch  
   → **Day-7 promo sheet appears** (`hasSeenOnboarding = true`; launch promo not yet shown)

7. **Tap "No thanks" on "Oh, before I forget..."; stop app; cold launch**  
   On the *"Oh, before I forget..."* contextual promo, tap **"No thanks"** → force-quit → wait ≥1 min → cold launch  
   → **Day-7 promo sheet appears** (`hasSeenOnboarding = true`)

---

#### B — Linear onboarding "Search & Duck.ai" path — search suggestion variant

To reach this state: complete linear onboarding → on the *"Want easy access to private AI chat in the address bar?"* screen select **"Toggle between Search and Duck.ai"** → tap **"Next"** → complete remaining steps.

Contextual onboarding starts with *"Ready to get started? Try a search or AI chat!"* dialog showing both search and Duck.ai suggestion chips.

8. **Select a search chip; "Try Visiting a Site" appears; stop app; cold launch**  
   On *"Ready to get started? Try a search or AI chat!"* select a **search suggestion chip** → SERP loads → *"That's DuckDuckGo Search!"* → *"Next, try visiting a site!"* dialog → force-quit → wait ≥1 min → cold launch  
   → **No promo sheet**

9. **Select a search chip; complete contextual onboarding to fire step; contextual promo pending; stop app; cold launch**  
   Select a search chip → complete *"Try Visiting a Site"* → fire tutorial → tap 🔥 Fire button → *"Oh, before I forget..."* appears → force-quit → cold launch  
   → **No promo sheet** (`subscriptionPromotionPending = true`)

10. **Select a search chip; complete full contextual onboarding including EOJ promo; stop app; cold launch**  
    Same as #9 but fully complete the contextual subscription promo (tap **"Try it free!"**, **"Learn More"**, or **"No thanks"**) → force-quit → cold launch  
    → **Day-7 promo sheet appears** (`hasSeenOnboarding = true`)

---

#### C — Linear onboarding "Search & Duck.ai" path — AI suggestion variant

11. **Select a Duck.ai chip; dialog visible; hide app, restore; tap X; stop app, cold launch**  
    On *"Ready to get started? Try a search or AI chat!"* select a **Duck.ai suggestion chip** → Duck.ai opens → contextual duck.ai dialog appears → press home button → switch back to DuckDuckGo → **No promo sheet** (foregrounding) → tap **X** → force-quit → wait ≥1 min → cold launch  
    → **Day-7 promo sheet appears** (before fire step; `subscriptionPromotionPending = false`)

12. **Select a Duck.ai chip; complete duck.ai dialogs; fire pending; stop app; cold launch**  
    Select Duck.ai chip → complete duck.ai contextual dialogs ("Got it!") → fire tutorial → tap 🔥 Fire button → *"Oh, before I forget..."* appears → force-quit → cold launch  
    → **No promo sheet** (`subscriptionPromotionPending = true`)

13. **Select a Duck.ai chip; complete full contextual onboarding; stop app; cold launch**  
    Same as #12 but fully complete the contextual subscription promo → force-quit → cold launch  
    → **Day-7 promo sheet appears** (`hasSeenOnboarding = true`)

---

#### D — Day-7 promo interaction flows (once promo appears)

14. **Tap "Try it free!" / "Learn More" CTA; verify destination; relaunch**  
    Trigger the day-7 promo (any flow above) → on the *"Did you know?"* sheet tap **"Try it free!"** (or **"Learn More"**) → Privacy Pro subscription page opens → force-quit → cold launch  
    → Subscription page opened with `funnel_modal_ios__subscriptionnudge` origin; **no promo sheet on next launch**

15. **Tap "No thanks" (dismiss button); relaunch**  
    Trigger promo → tap **"No thanks"** → force-quit → cold launch  
    → **No promo sheet** (already shown)

16. **Tap ✕ (close icon); relaunch**  
    Trigger promo → tap the **✕ icon** (top-right) → force-quit → cold launch  
    → **No promo sheet** (already shown)

17. **Hide app while promo is visible; reopen; then cold launch**  
    Trigger promo → press home button without dismissing → switch back to DuckDuckGo → force-quit → cold launch  
    → Promo was visible on reopen; **no promo sheet on next cold launch** (already shown)

---

#### E — Edge cases / negative flows

18. **Cooldown not passed: launch immediately after install**  
    Fresh install → enable flags → complete linear + contextual onboarding → cold launch immediately (< 1 min since install)  
    → **No promo sheet** (cooldown not met)

19. **`subscriptionPromoForExistingUsers` flag disabled**  
    Disable `subscriptionPromoForExistingUsers` flag → complete onboarding → wait ≥1 min → cold launch  
    → **No promo sheet**

20. **`privacyProOnboardingPromotion` flag disabled**  
    Disable `privacyProOnboardingPromotion` → same setup  
    → **No promo sheet**

21. **WinBack-eligible user: WinBack promo takes priority**  
    Set up a user qualifying for both WinBack offer and day-7 promo → cold launch  
    → **WinBack offer sheet appears** (higher priority); day-7 promo stays pending for the next launch after WinBack is dismissed

22. **Reinstaller-eligible user: reinstaller promo takes priority**  
    User who skipped linear onboarding (tap X → "Start Browsing") + `subscriptionPromoForReinstallers` enabled → cold launch  
    → **Reinstaller promo sheet appears** (higher priority)

---

#### F — Other modal prompts not regressed by per-provider gate refactor

23. **Default Browser prompt does not appear before onboarding is complete**  
    User who has NOT completed contextual onboarding (`hasSeenOnboarding = false`, `subscriptionPromotionPending = false`, `subscriptionPromoForExistingUsers` disabled to prevent that promo) → cold launch  
    → **No Default Browser prompt** (default `isEligibleToPresent` still requires onboarding complete)  
    *(Validates the per-provider gate refactor hasn't changed behaviour for other providers)*

---

#### G — Upgrade scenario

24. **Upgrade from pre-feature build; onboarding complete; promo shown on first launch after update**  
    Install build at commit `0f064b1e66` → complete linear + contextual onboarding all the way through the *"Oh, before I forget..."* contextual promo → update app to `alex/subscription-day7-promo` with flags enabled → cold launch (install date is ≥1 min ago)  
    → **Day-7 promo sheet appears** (`hasSeenOnboarding = true` carried over; `subscriptionPromotionDialogSeen` = false since it's a new UserDefaults key; cooldown already passed)

25. **Upgrade from pre-feature build; contextual onboarding incomplete; no promo on first launch after update**  
    Install build at commit `0f064b1e66` → complete linear onboarding only (stop before completing contextual dialogs) → update to `alex/subscription-day7-promo` → cold launch (install date ≥1 min ago)  
    → **No promo sheet**

---

### Impact and Risks

**Impact Level: Medium**

#### What could go wrong?

- **Promo appears during active contextual onboarding** — would create two overlapping subscription-related sheets. Mitigated by the `isEligibleToPresent` gate checking `isShowingContextualOnboardingDialog`; covered by testing flows #5, #9, #12.
- **Promo shown twice** — duplicate impression if `subscriptionPromotionDialogSeen` is not persisted correctly. Mitigated by setting the flag in `markLaunchPromptPresented()` before presenting; covered by flows #14–16.
- **Other modal prompts regressed by gate refactor** — existing providers could break if the per-provider default gate behaves differently from the removed global gate. The default `isEligibleToPresent` returns `isOnboardingComplete`, which is functionally equivalent; covered by flow #23.
- **Cooldown bypassed by date manipulation** — low risk for production; the debug override (1-min) must be reverted before shipping.

### Quality Considerations

- **Privacy**: No new user data collected. Pixels carry only `returningUser` and `freeTrial` boolean parameters, consistent with existing subscription promo pixels.
- **Performance**: Eligibility check is synchronous and lightweight; no network calls added to the launch critical path.
- **Feature flag gating**: Double-gated (`subscriptionPromoForExistingUsers` + `privacyProOnboardingPromotion`), enabling gradual rollout and instant kill-switch.
- **Cooldown override**: The 1-minute override in `hasCooldownPassed()` must be reverted to the 7-day logic (`cooldownDays = 7`) before the flag is enabled in production. The TODO comment marks it clearly.
- **Funnel origin**: Uses a new distinct origin value (`funnel_modal_ios__subscriptionnudge`) so this promo's conversion can be tracked independently from the reinstaller and skipped-onboarding promos.
- **Test coverage**: `SubscriptionPromoExistingUserCoordinatorTests` covers eligibility logic, pixel firing, CTA navigation, and dismiss handling.

### Notes to Reviewer

- The `isEligibleToPresent(isOnboardingComplete:)` protocol method replaces the old global service-level `hasSeenOnboarding` guard. The net behaviour for existing providers is identical — they now check onboarding state per-provider via the default implementation rather than a single service gate. The only change in observable behaviour is that `SubscriptionPromoExistingUserCoordinator` can present to users who haven't fully completed contextual onboarding, as long as `isShowingContextualOnboardingDialog` is false.
- `hasAppStoreProductsAvailable` check is commented out in `shouldPresentLaunchPrompt()` — intentional for the current testing phase; evaluate whether to re-enable before rollout.
- The reinstaller promo coordinator (`SubscriptionPromoCoordinator`) and the existing-user coordinator share `SubscriptionPromoCoordinating` and `SubscriptionPromoModalPromptProvider`, keeping the sheet UI consistent.