Dialog implementations:
1. CheckingDialog - Animated progress while checking for updates (10s timeout)
2. UpdateAvailableDialog - Shows version, changelog, action buttons
3. DownloadingDialog - Progress bar with size display and cancel option
4. InstallDialog - Confirmation with unsaved changes warning
5. NoUpdateDialog - Clean confirmation when up-to-date
6. ErrorDialog - Error handling with retry and manual download options
Test coverage:
- 29 unit tests for all 6 dialogs
- 100% coverage of update_manager_ui.py
- Signal emission testing for all interactive elements
- Progress bar and file display functionality
- Dialog state and flags validation
