Add YouTube download feature with yt-dlp integration

This commit implements a comprehensive YouTube download feature that allows
administrators to download audio from YouTube videos and playlists.

Backend Changes:
- Add yt-dlp-wrap npm dependency for YouTube downloading
- Create YouTubeDownloadManager to handle download queue and execution
- Create YouTubeDownloadController with API endpoints for download/queue/cancel
- Add YouTube utility functions for URL validation and metadata extraction
- Create YouTubeDownload object model for tracking download state
- Add API routes: POST /api/youtube/download, GET /api/youtube/queue, DELETE /api/youtube/download/:id
- Implement playlist support - automatically queue all videos from playlist URLs
- Download audio in MP3 format with best quality
- Organize files in subfolder structure: <Uploader>/<Video Title>/
- Extract metadata and create library items automatically
- Real-time progress updates via Socket.io

Frontend Changes:
- Create YouTubeDownloadModal component for user-friendly download interface
- Add download button to Appbar header (admin-only, visible with current library)
- Update Vuex store with modal state management
- Register modal globally in default layout
- Implement Socket.io listeners for download events (started, progress, completed, failed, queued)
- Show toast notifications for download status updates

Features:
- Admin-only access control
- Support for single videos and playlists
- Audio quality selection (best, 320kbps, 256kbps, 192kbps, 128kbps)
- Library and folder selection
- Download queue management
- Real-time progress tracking
- Automatic thumbnail download as cover image
- Automatic library item creation with metadata
- Error handling and user feedback

Technical Details:
- Uses yt-dlp for reliable YouTube downloading
- Integrates with existing task management system
- Respects library folder permissions
- Follows existing code patterns (similar to podcast downloads)
- Socket.io events for real-time updates

package.json

@@ -57,7 +57,8 @@
     "socket.io": "^4.5.4",
     "sqlite3": "^5.1.7",
     "ssrf-req-filter": "^1.1.0",
-    "xml2js": "^0.5.0"
+    "xml2js": "^0.5.0",
+    "yt-dlp-wrap": "^2.3.12"
   },
   "devDependencies": {
     "chai": "^4.3.10",