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/NoteEditor.tsx

@@ -37,7 +37,7 @@ export function NoteEditor({ note, onUpdateNote, onUnsavedChanges, categories, i
   const [processedContent, setProcessedContent] = useState('');
   const [isLoadingImages, setIsLoadingImages] = useState(false);
   const [isUploading, setIsUploading] = useState(false);
-  const previousNoteIdRef = useRef<number | null>(null);
+  const previousNoteIdRef = useRef<number | string | null>(null);
   const previousNoteContentRef = useRef<string>('');
   const textareaRef = useRef<HTMLTextAreaElement>(null);
   const fileInputRef = useRef<HTMLInputElement>(null);