Prevent note switching when there are unsaved changes

- Added hasUnsavedChanges state tracking in App.tsx
- NoteEditor now notifies parent via onUnsavedChanges callback
- NotesList receives hasUnsavedChanges prop
- Clicking on other notes is prevented when current note has unsaved changes
- Visual feedback: other notes become semi-transparent with cursor-not-allowed
- Tooltip shows 'Save current note before switching' on disabled notes
- Much simpler and more reliable than trying to auto-save on switch

src/components/NoteEditor.tsx

@@ -11,6 +11,7 @@ interface NoteEditorProps {
   note: Note | null;
   onUpdateNote: (note: Note) => void;
   fontSize: number;
+  onUnsavedChanges?: (hasChanges: boolean) => void;
 }
 
 const turndownService = new TurndownService({
@@ -18,7 +19,7 @@ const turndownService = new TurndownService({
   codeBlockStyle: 'fenced',
 });
 
-export function NoteEditor({ note, onUpdateNote, fontSize }: NoteEditorProps) {
+export function NoteEditor({ note, onUpdateNote, fontSize, onUnsavedChanges }: NoteEditorProps) {
   const [localTitle, setLocalTitle] = useState('');
   const [localFavorite, setLocalFavorite] = useState(false);
   const [isSaving, setIsSaving] = useState(false);
@@ -26,6 +27,11 @@ export function NoteEditor({ note, onUpdateNote, fontSize }: NoteEditorProps) {
   const [titleManuallyEdited, setTitleManuallyEdited] = useState(false);
   const previousNoteIdRef = useRef<number | null>(null);
 
+  // Notify parent component when unsaved changes state changes
+  useEffect(() => {
+    onUnsavedChanges?.(hasUnsavedChanges);
+  }, [hasUnsavedChanges, onUnsavedChanges]);
+
   const editor = useEditor({
     extensions: [
       StarterKit,