feat: implement optimized Undo for delete and move actions

- Added UndoService with 10-action history stack.
- Integrated Undo Snackbar into EmailListScreen and EmailDetailScreen.
- Added EmailRepository.cancelPendingChange to optimize undo by removing
  unsynced local mutations.
- Fixed sorting bug in compareMailboxes for unknown roles.
- Increased unit coverage to 83% with new model and utility tests.
- Verified with full test suite (task check).

next.md

@@ -20,11 +20,54 @@ Then push
 
 ## Tasks
 
-### 1. Implement Undo for Delete and Move actions
+### 1. Infrastructure Hardening and Code Quality
 
-Provide a way for users to undo accidental deletions or moves, improving the safety of the application.
+Continue the momentum from the safety hardening and infrastructure work.
+The focus is on making the app ready for real-world use with robust error
+handling and performance optimizations.
 
-- **Infrastructure**: Implement a `ChangeLog` or similar mechanism to track the last N destructive actions.
-- **UI**: Display a snackbar with an "Undo" button after a delete or move action.
-- **Logic**: Implement the reverse operation (moving back from Trash or to the source folder) when Undo is pressed.
-- **Sync**: Ensure that undo operations correctly interact with the `pending_changes` queue.
+- **Sync Reliability**: Implement a "Sync reliability" check that compares local
+  counts with server counts periodically.
+- **Search Optimization**: Add a "Recent Searches" history and optimize local search
+  indexing for large accounts.
+- **UI Polishing**: Ensure consistent spacing and theming across all screens.
+- **Error Boundaries**: Add more granular error boundaries to prevent the entire
+  app from crashing if a single widget fails.
+
+### 2. Unified Attachment Handling and Improved Previews
+
+Refactor attachment logic to be more consistent and provide better user feedback.
+
+- **Previews**: Implement thumbnail generation for image attachments.
+- **Progress**: Show download progress in the UI when fetching attachments.
+- **Caching**: Implement a more robust caching mechanism with expiry.
+
+### 3. Sync Reliability and Conflict Resolution
+
+- **Reliability**: Implement a "Reliability Runner" that periodically verifies local state against the server.
+- **Conflicts**: Improve handling of concurrent changes (e.g., mail moved on server while local move is pending).
+
+### 4. Advanced Search and Performance
+
+- **Indexing**: Optimize database indexes for search performance.
+- **UI**: Add advanced search filters (date range, attachment size, etc.).
+
+### 5. Multi-account Sync and Reliability Runner
+
+Implement a robust verification system to ensure the local database accurately
+reflects the server state across multiple accounts and protocols.
+
+- **Reliability Runner**: A background service that periodically fetches a "ground truth"
+  snapshot (UIDs/IDs only) for all folders and identifies discrepancies.
+- **Sync Reliability UI**: Add a "Sync health" indicator in settings showing when each
+  account was last verified 100%.
+- **Network Resilience**: Improve backoff and retry logic for intermittent connections,
+  especially for mobile.
+- **Fuzz Testing**: Add a basic fuzz test for the sync engine to handle simulated
+  real-world network latency and RFC edge cases.
+
+### 6. Coverage Gate Maintenance
+
+Reduce the `_excluded` list in `scripts/check_coverage.dart`.
+Add a test to ensure the exclusion list doesn't contain files that no longer
+exist ("ghost paths").