- 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).