perf: fix token listener accumulation, add request deduplication, network-aware cache warming, and offline banner (#812 #813 #814 #824)

[BigBen-7]

Closes #812
Closes #813
Closes #814
Closes #824

Summary

Four performance and UX improvements addressing offline behaviour, startup efficiency, and duplicate request handling.

#812 — Token expiry listener accumulation
pushNotifications.ts had no guard preventing multiple Notifications.addPushTokenListener registrations across app launches and background restores. Added a module-level tokenExpirySubscription variable and a setupTokenExpiryListener() export that skips registration if a listener is already active (early-return instead of remove-and-readd). The same skip-if-registered pattern was applied to setupForegroundBadgeSync() to prevent badge handlers from doubling up on re-launch.

#813 — Request deduplication in RequestQueue
requestQueue.ts enqueued every call unconditionally, so submitting a form while offline multiple times (or processing the queue more than once) sent the same mutation to the backend multiple times. Added a fingerprint(config) method that hashes method + URL + body into a deterministic key, stored as fingerprint on QueuedRequest. addToQueue now checks for an existing entry with the same fingerprint before enqueuing, and returns the existing request's id when a duplicate is detected.

#814 — Network-aware cache warming
warmCriticalCaches() previously fetched all data unconditionally, causing multi-second startup latency on 2G/3G and potential data exhaustion. The function now reads Network.getNetworkStateAsync() first:

• Offline: skipped entirely
• Cellular (non-WiFi): minimal prefetch — courses only, user profile skipped
• WiFi / OTHER: full prefetch (original behaviour)

#824 — Offline mode banner
Added src/components/common/OfflineBanner.tsx: an Animated banner that slides in from the top within 500 ms of going offline, shows the number of pending queued actions (e.g. "2 actions pending sync"), and slides out automatically on reconnection. Subscribes to requestQueue.onPendingCountChange for live queue depth. Accessible via accessibilityRole="alert" and accessibilityLiveRegion="assertive".

Files changed

• src/services/pushNotifications.ts — token listener guard + skip-if-registered ([Performance] pushNotifications.ts token expiry listener accumulates on each app launch #812)
• src/services/api/requestQueue.ts — fingerprint-based deduplication ([Performance] requestQueue.ts has no request deduplication — identical mutations fire multiple times #813)
• src/services/cacheWarming.ts — network-quality gate ([Performance] warmCriticalCaches() ignores network quality — prefetches full data on slow connections #814)
• src/components/common/OfflineBanner.tsx — new offline indicator ([Enhancement] No offline mode indicator — users have no feedback when app is in degraded state #824)

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