Offline read mode + offline banner

[fboucher]

Parent PRD

#110

Depends on

#123 (online-first data layer must exist)

Summary

Implement the offline read path in OfflineDataService and add the persistent offline banner. When offline, all reads route to local SQLite. Mark-as-read works offline (queued). Search by name and filter by read/unread status work from local cache. Settings accessible offline.

OfflineDataService — offline path

• Detect Connectivity.NetworkAccess != NetworkAccess.Internet
• All read operations delegate to ILocalDataService
• Mark-as-read sets IsPendingSync = true locally

Offline banner

• Persistent banner at top of app when offline
• Clearly labels offline state
• Disappears when connectivity restored

Offline search + filter

• Text search filters local post list by name in real time
• Read/unread filter applies to local cache

Acceptance Criteria

• OfflineDataService routes reads to SQLite when offline
• Offline banner visible whenever Connectivity.NetworkAccess != Internet
• Banner disappears when back online
• Mark-as-read works offline (queued via IsPendingSync)
• Search and read/unread filter work from local cache
• Settings page accessible offline
• Summary list accessible offline (from local cache)

Stories from PRD

User stories 2, 3, 6, 9, 12, 13, 14, 15, 16

[fboucher]

Triage: No changes needed

Unaffected by the auth removal decision. This is purely about connectivity-aware data routing and offline UI (banner, search, filters). Keeps its dependency on #123.