Implement autosave logic

[ItzSouraseez]

PR Description

This PR implements an autosave on close feature that automatically saves all unsaved changes when the application closes, providing a modern, seamless user experience while maintaining backward compatibility with the existing prompt-based save mechanism.

Key Changes:

• Added autoSaveOnExit boolean field to SettingsModel (defaults to true)
• Updated ThemeStateNotifier to support the new setting
• Added "Auto-save on Exit" checkbox in Settings UI
• Refactored onWindowClose() handler with priority-based logic:
  1. Auto-save if enabled and has unsaved changes
  2. Show prompt dialog if auto-save disabled but prompt enabled
  3. Close immediately if no unsaved changes or both settings disabled
• Added error handling to prevent data loss when save operations fail

User Experience:

• By default, the app will auto-save on exit without showing a dialog
• Users can disable auto-save and keep the traditional "Save Changes" prompt
• Users can disable both for immediate closure without saving
• If auto-save fails, an error dialog appears with retry and close-without-saving options

Technical Notes:

• Fully backward compatible with existing user settings
• Gracefully handles missing autoSaveOnExit field in stored settings (defaults to true)
• Leverages existing saveData() method which handles both requests and environments
• Cross-platform compatible (macOS, Windows, Linux)

Related Issues

• Closes An autosave on close? #993

Checklist

• I have gone through the contributing guide
• I have updated my branch and synced it with project main branch before making this PR
• I am using the latest Flutter stable branch (run flutter upgrade and verify)
• I have run the tests (flutter test) and all tests are passing

Added/updated tests?

We encourage you to add relevant test cases.

• Yes
• No, and this is why: The changes primarily affect UI behavior and window lifecycle events which are difficult to unit test. Manual testing has been performed across different setting combinations. Unit tests for SettingsModel serialization/deserialization would be a valuable addition in a follow-up PR.

OS on which you have developed and tested the feature?

• Windows
• macOS
• Linux