feat: WebDAV file access and category color sync (v0.2.0)

Major Changes:
- Switch from Nextcloud Notes API to direct WebDAV file access
- Notes stored as .txt files with filename-based IDs for reliability
- Implement safer sync strategy without clearNotes() to prevent data loss
- Add ETag-based conflict detection for concurrent edits
- Add category color sync to .category-colors.json on server
- Show neutral gray badges for categories without assigned colors

Technical Improvements:
- Replace numeric IDs with filename-based string IDs
- Update Note type to support both number and string IDs
- Implement WebDAV methods: fetchNotesWebDAV, createNoteWebDAV, updateNoteWebDAV, deleteNoteWebDAV
- Add CategoryColorsSync service for server synchronization
- Remove hash-based color fallback (only show colors when explicitly set)

Bug Fixes:
- Fix category badge rendering to show all categories
- Prevent note loss during sync operations
- Improve offline-first functionality with better merge strategy

src/db/localDB.ts

@@ -59,7 +59,7 @@ class LocalDB {
     });
   }
 
-  async getNote(id: number): Promise<Note | undefined> {
+  async getNote(id: number | string): Promise<Note | undefined> {
     return new Promise((resolve, reject) => {
       const store = this.getStore(NOTES_STORE);
       const request = store.get(id);
@@ -89,7 +89,7 @@ class LocalDB {
     });
   }
 
-  async deleteNote(id: number): Promise<void> {
+  async deleteNote(id: number | string): Promise<void> {
     return new Promise((resolve, reject) => {
       const store = this.getStore(NOTES_STORE, 'readwrite');
       const request = store.delete(id);