feat(sync): add cross-platform sync managers for iOS, Android, and Web
- iOS: PlatformSyncManager — URLSession, delta sync, batch upload, offline queue, HMAC auth
- Android: PlatformApiClient + SyncRepository — HttpURLConnection, Room integration, offline queue
- Web: platform-sync.ts + use-sync.ts — fetch client, localStorage queue, React hook with 60s auto-sync
All consume platform-service /timers/*, /routines/*, /households/* endpoints.
Sync protocol: syncVersion optimistic concurrency, delta sync via ?since=, batch upsert for offline flush.
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