feat(device-sync): USB/SD card sync page (WIP)

Adds a new Device Sync page for transferring music from Navidrome to
USB drives or SD cards. Sidebar entry is hidden by default.

- Four new Tauri commands: sync_track_to_device, compute_sync_paths,
  list_device_dir_files, delete_device_file
- Filename template engine with cross-platform path sanitization
- 4-concurrent-worker download, live progress via device:sync:progress event
- Persistent source list (albums/playlists/artists) with checkbox deletion
- Expandable artist tree in browser panel (per-album selection)
- i18n: DE + EN complete; other locales have stub keys

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/locales/en.ts

@@ -25,6 +25,7 @@ export const enTranslation = {
     mostPlayed: 'Most Played',
     radio: 'Internet Radio',
     folderBrowser: 'Folder Browser',
+    deviceSync: 'Device Sync',
     libraryScope: 'Library scope',
     allLibraries: 'All libraries',
     expandPlaylists: 'Expand playlists',
@@ -1023,4 +1024,35 @@ export const enTranslation = {
     empty: 'Empty folder',
     error: 'Failed to load',
   },
+  deviceSync: {
+    title: 'Device Sync',
+    targetFolder: 'Target Folder',
+    noFolderChosen: 'No folder chosen',
+    chooseFolder: 'Choose…',
+    filenameTemplate: 'Filename Template',
+    templateHint: 'Variables: {artist}, {album}, {title}, {track_number}, {disc_number}, {year}',
+    onDevice: 'On Device',
+    addSources: 'Add…',
+    colName: 'Name',
+    colType: 'Type',
+    syncResult: '{{done}} transferred, {{skipped}} already up to date ({{total}} total)',
+    deleteFromDevice: 'Delete from device ({{count}})',
+    confirmDelete: 'Delete {{count}} item(s) from the device: {{names}}?',
+    deleteComplete: '{{count}} item(s) removed from device.',
+    selectedSources: 'Selected Sources',
+    noSourcesSelected: 'No sources selected. Click items in the browser to add them.',
+    clearAll: 'Clear all',
+    tabPlaylists: 'Playlists',
+    tabAlbums: 'Albums',
+    tabArtists: 'Artists',
+    searchPlaceholder: 'Search…',
+    syncButton: 'Sync to Device',
+    cancel: 'Cancel',
+    noTargetDir: 'Please choose a target folder first.',
+    noSources: 'Please select at least one source.',
+    noTracks: 'No tracks found in the selected sources.',
+    fetchError: 'Failed to fetch tracks from server.',
+    syncComplete: 'Sync complete.',
+    dismiss: 'Dismiss',
+  },
 };