Added a new visual Appearance selector and new Paper Themes:
- Replaced the basic dropdown menu with a visual Grid selector for Paper Themes, showing live miniature snippets of how your notes will look.
- Added a new 'Large Grid' paper background option.
- 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.
- Added an interactive 'Undo' button directly to the note deletion popup toast for immediate and reliable recovery.
- 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".
