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).

.gitignore

@@ -58,3 +58,5 @@ linux/flutter/generated_plugins.cmake
 .claude
 
 .task
+
+*.log