End-to-end sync testing + error handling

[tomymaritano]

Context

Sync infrastructure exists but hasn't been tested end-to-end. Several error handling gaps identified.

Test Matrix

1. Auth round-trip: magic link → verify → JWT → session persistence
2. Push: create note locally → sync → verify on server
3. Pull: modify on server → pull → verify local
4. Conflict: edit same note on two devices → conflict UI appears
5. Offline: go offline → edit → reconnect → queued sync executes
6. Encryption: verify content encrypted in API payloads

Known Issues

• syncService.ts — auto-sync starts but no error handling on failure
• Token refresh on 401 may not retry original request
• Device ID generation and persistence unclear
• No user-visible feedback when sync fails silently

Deliverables

• All 6 test scenarios passing
• Sync error handling (no silent failures)
• Offline queue visibility in status bar
• Sync history accessible to user
• Conflict resolution UX tested