add view mode as default, open new notes in edit mode with cursor ready

- Introduce NoteMode enum { view, edit, preview } replacing the old bool
- Default mode when opening a note is view (syntax-highlighted, read-only)
- Menu now shows all three modes with a checkmark on the active one
- Save and autosave menu items disabled outside edit mode
- New notes open in edit mode with cursor positioned after the header
  and keyboard focused, ready to type

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/note_list.dart

@@ -449,6 +449,7 @@ Happy note taking! ✨
               note: newNote,
               isDarkMode: widget.isDarkMode,
               notesDirectoryPath: notesDirectoryPath,
+              initialMode: NoteMode.edit,
             ),
           ),
         );