Added new Quality of Life features and fixed critical bugs:
- Added a visual Theme Switcher in the main menu to easily toggle between System, Light, and Dark modes.
- Added a Startup Behavior preference to choose whether the app restores your last active note or starts with a fresh empty draft.
- Fixed a race condition where quickly deleting a note using a keyboard shortcut would occasionally cause the note to save itself in the background as a "ghost note".
