fix: remove App Tracking Transparency (ATT) from native analytics flow

- Drop `expo-tracking-transparency` dependency and plugin config from `app.json`
- Replace `applyTrackingTransparency` with `initAnalytics()` that simply syncs PostHog opt-in/out from the stored preference on startup
- Remove ATT permission prompt, IDFA/AAID identification, and one-time migration logic from `posthog.ts`
- Analytics now starts disabled on both iOS and Android; user opts in via Settings toggle
- Update telemetry docs and privacy policy to reflect the simplified consent flow (no ATT prompt, no advertising IDs)