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/zh.ts

@@ -24,6 +24,7 @@ export const zhTranslation = {
     mostPlayed: '最常播放',
     radio: '网络电台',
     folderBrowser: '文件夹浏览器',
+    deviceSync: '设备同步',
     libraryScope: '资料库范围',
     allLibraries: '所有资料库',
     expandPlaylists: '展开播放列表',
@@ -1012,4 +1013,31 @@ export const zhTranslation = {
     empty: '空文件夹',
     error: '加载失败',
   },
+  deviceSync: {
+    title: '设备同步',
+    targetFolder: '目标文件夹',
+    noFolderChosen: '未选择文件夹',
+    chooseFolder: '选择…',
+    filenameTemplate: '文件名模板',
+    templateHint: '变量：{artist}、{album}、{title}、{track_number}、{disc_number}、{year}',
+    cleanupButton: '删除不在选择范围内的文件',
+    cleanupNothingToDelete: '无需删除 — 设备已同步。',
+    confirmCleanup: '将从设备中删除 {{count}} 个不在当前选择范围内的文件。继续？',
+    cleanupComplete: '已从设备中删除 {{count}} 个文件。',
+    selectedSources: '已选来源',
+    noSourcesSelected: '未选择来源。点击浏览器中的项目以添加。',
+    clearAll: '全部清除',
+    tabPlaylists: '播放列表',
+    tabAlbums: '专辑',
+    tabArtists: '艺术家',
+    searchPlaceholder: '搜索…',
+    syncButton: '同步到设备',
+    cancel: '取消',
+    noTargetDir: '请先选择目标文件夹。',
+    noSources: '请至少选择一个来源。',
+    noTracks: '所选来源中未找到曲目。',
+    fetchError: '从服务器加载曲目失败。',
+    syncComplete: '同步完成。',
+    dismiss: '关闭',
+  },
 };