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

server/utils/youtubeUtils.js

@@ -0,0 +1,144 @@
+const Logger = require('../Logger')
+
+/**
+ * Validate if a URL is a valid YouTube URL
+ * Supports: youtube.com, youtu.be, music.youtube.com
+ *
+ * @param {string} url
+ * @returns {boolean}
+ */
+function isValidYouTubeUrl(url) {
+  if (!url || typeof url !== 'string') return false
+
+  try {
+    const urlObj = new URL(url)
+    const hostname = urlObj.hostname.toLowerCase()
+
+    // Support various YouTube domains
+    const validHosts = [
+      'www.youtube.com',
+      'youtube.com',
+      'youtu.be',
+      'music.youtube.com',
+      'm.youtube.com'
+    ]
+
+    if (!validHosts.includes(hostname)) return false
+
+    // Check for video ID or playlist ID
+    if (hostname === 'youtu.be') {
+      return urlObj.pathname.length > 1
+    }
+
+    return urlObj.searchParams.has('v') || urlObj.searchParams.has('list') || urlObj.pathname.includes('/watch')
+  } catch (error) {
+    Logger.error('[youtubeUtils] Invalid URL:', error)
+    return false
+  }
+}
+
+/**
+ * Extract video ID from YouTube URL
+ *
+ * @param {string} url
+ * @returns {string|null}
+ */
+function extractVideoId(url) {
+  try {
+    const urlObj = new URL(url)
+    const hostname = urlObj.hostname.toLowerCase()
+
+    if (hostname === 'youtu.be') {
+      return urlObj.pathname.slice(1).split('?')[0]
+    }
+
+    return urlObj.searchParams.get('v')
+  } catch (error) {
+    Logger.error('[youtubeUtils] Failed to extract video ID:', error)
+    return null
+  }
+}
+
+/**
+ * Check if URL is a playlist
+ *
+ * @param {string} url
+ * @returns {boolean}
+ */
+function isPlaylistUrl(url) {
+  try {
+    const urlObj = new URL(url)
+    return urlObj.searchParams.has('list') || urlObj.pathname.includes('/playlist')
+  } catch (error) {
+    return false
+  }
+}
+
+/**
+ * Extract playlist ID from YouTube URL
+ *
+ * @param {string} url
+ * @returns {string|null}
+ */
+function extractPlaylistId(url) {
+  try {
+    const urlObj = new URL(url)
+    return urlObj.searchParams.get('list')
+  } catch (error) {
+    Logger.error('[youtubeUtils] Failed to extract playlist ID:', error)
+    return null
+  }
+}
+
+/**
+ * Sanitize title for filename
+ * Remove invalid characters and limit length
+ *
+ * @param {string} title
+ * @returns {string}
+ */
+function sanitizeTitleForFilename(title) {
+  if (!title) return 'untitled'
+
+  // Remove invalid filename characters
+  let sanitized = title.replace(/[<>:"/\\|?*\x00-\x1f]/g, '')
+
+  // Replace multiple spaces with single space
+  sanitized = sanitized.replace(/\s+/g, ' ').trim()
+
+  // Limit length to 200 characters
+  if (sanitized.length > 200) {
+    sanitized = sanitized.substring(0, 200).trim()
+  }
+
+  return sanitized || 'untitled'
+}
+
+/**
+ * Format duration from seconds to readable format
+ *
+ * @param {number} seconds
+ * @returns {string}
+ */
+function formatDuration(seconds) {
+  if (!seconds || isNaN(seconds)) return '0:00'
+
+  const hours = Math.floor(seconds / 3600)
+  const minutes = Math.floor((seconds % 3600) / 60)
+  const secs = Math.floor(seconds % 60)
+
+  if (hours > 0) {
+    return `${hours}:${minutes.toString().padStart(2, '0')}:${secs.toString().padStart(2, '0')}`
+  }
+
+  return `${minutes}:${secs.toString().padStart(2, '0')}`
+}
+
+module.exports = {
+  isValidYouTubeUrl,
+  extractVideoId,
+  isPlaylistUrl,
+  extractPlaylistId,
+  sanitizeTitleForFilename,
+  formatDuration
+}