feat: implement offline-first subscription sync layer with CRDT

[rindicomfort]

Resolves #665

Context

This pull request introduces a robust offline-first synchronization layer for SubTrackr subscription data management. It allows users to create, update, delete, and toggle subscriptions offline, with automatic deterministic state merging and conflict resolution on network recovery.

Implementation Details

1. NetworkMonitor (src/services/network/networkMonitor.ts):

   • Monitors connectivity events using @react-native-community/netinfo.
   • Exposes listener and query interfaces for online status.

2. SubscriptionCRDT (src/services/cache/crdt.ts):

   • Implements a State-based LWW-Element-Set + property-level LWW-Register CRDT merge engine.
   • Deterministically merges divergent subscription states, managing additions, modifications, and tombstoned deletions.

3. useSubscriptionStore Integration (src/store/subscriptionStore.ts):

   • Extends Zustand state with syncStatus ('idle' | 'pending' | 'syncing' | 'conflict' | 'error') and crdtMetadata.
   • Automatically timestamps modifications and schedules synchronization immediately if online.
   • Implements persistent AsyncStorage serialization for CRDT metadata.

4. useOfflineSync Hook (src/hooks/useOfflineSync.ts):

   • Monitors network transitions to auto-trigger synchronization upon connection recovery.
   • Utilizes exponential backoff for failed/transient retry attempts (starting at 1s, capped at 60s).

5. Test Coverage:

   • src/services/cache/__tests__/crdt.test.ts (covers property-level merging, deletion priorities, tombstones, and algebraic correctness).
   • src/hooks/__tests__/useOfflineSync.test.ts (covers hook recovery triggers and exponential backoff retry cycles).

6. Documentation:

   • Added complete architecture and sync guarantees design doc in docs/offline-architecture.md.

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