feat: switch from Notes API to WebDAV file access

- Replace API-based note operations with direct WebDAV file access
- Use filename-based IDs instead of numeric IDs for better reliability
- Implement safer merge strategy that doesn't clear local notes
- Add ETag-based conflict detection to prevent data loss
- Support string | number IDs throughout the codebase
- Notes are now stored as .txt files in /Notes/{category}/
- Eliminates race conditions and temporary ID conflicts
- More reliable sync with direct file system access

src/components/NotesList.tsx

@@ -4,8 +4,8 @@ import { SyncStatus } from '../services/syncManager';
 
 interface NotesListProps {
   notes: Note[];
-  selectedNoteId: number | null;
-  onSelectNote: (id: number) => void;
+  selectedNoteId: number | string | null;
+  onSelectNote: (id: number | string) => void;
   onCreateNote: () => void;
   onDeleteNote: (note: Note) => void;
   onSync: () => void;
@@ -36,7 +36,7 @@ export function NotesList({
   isOnline,
 }: NotesListProps) {
   const [isSyncing, setIsSyncing] = React.useState(false);
-  const [deleteClickedId, setDeleteClickedId] = React.useState<number | null>(null);
+  const [deleteClickedId, setDeleteClickedId] = React.useState<number | string | null>(null);
   const [width, setWidth] = React.useState(() => {
     const saved = localStorage.getItem('notesListWidth');
     return saved ? parseInt(saved, 10) : 320;