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/store/deviceSyncStore.ts

@@ -0,0 +1,89 @@
+import { create } from 'zustand';
+import { persist } from 'zustand/middleware';
+
+export interface DeviceSyncSource {
+  type: 'album' | 'playlist' | 'artist';
+  id: string;
+  name: string;
+}
+
+export interface DeviceSyncJob {
+  id: string;
+  total: number;
+  done: number;
+  skipped: number;
+  failed: number;
+  status: 'running' | 'done' | 'cancelled';
+}
+
+interface DeviceSyncState {
+  targetDir: string | null;
+  filenameTemplate: string;
+  sources: DeviceSyncSource[];        // persistent device content list
+  checkedIds: string[];               // currently checked for deletion (not persisted)
+  activeJob: DeviceSyncJob | null;
+
+  setTargetDir: (dir: string | null) => void;
+  setFilenameTemplate: (t: string) => void;
+  addSource: (source: DeviceSyncSource) => void;
+  removeSource: (id: string) => void;
+  clearSources: () => void;
+  toggleChecked: (id: string) => void;
+  setCheckedIds: (ids: string[]) => void;
+  setActiveJob: (job: DeviceSyncJob | null) => void;
+  updateJob: (update: Partial<DeviceSyncJob>) => void;
+}
+
+export const useDeviceSyncStore = create<DeviceSyncState>()(
+  persist(
+    (set) => ({
+      targetDir: null,
+      filenameTemplate: '{artist}/{album}/{track_number} - {title}',
+      sources: [],
+      checkedIds: [],
+      activeJob: null,
+
+      setTargetDir: (dir) => set({ targetDir: dir }),
+      setFilenameTemplate: (t) => set({ filenameTemplate: t }),
+
+      addSource: (source) =>
+        set((s) => ({
+          sources: s.sources.some((x) => x.id === source.id)
+            ? s.sources
+            : [...s.sources, source],
+        })),
+
+      removeSource: (id) =>
+        set((s) => ({
+          sources: s.sources.filter((x) => x.id !== id),
+          checkedIds: s.checkedIds.filter((x) => x !== id),
+        })),
+
+      clearSources: () => set({ sources: [], checkedIds: [] }),
+
+      toggleChecked: (id) =>
+        set((s) => ({
+          checkedIds: s.checkedIds.includes(id)
+            ? s.checkedIds.filter((x) => x !== id)
+            : [...s.checkedIds, id],
+        })),
+
+      setCheckedIds: (ids) => set({ checkedIds: ids }),
+
+      setActiveJob: (job) => set({ activeJob: job }),
+
+      updateJob: (update) =>
+        set((s) => ({
+          activeJob: s.activeJob ? { ...s.activeJob, ...update } : null,
+        })),
+    }),
+    {
+      name: 'psysonic_device_sync',
+      partialize: (s) => ({
+        targetDir: s.targetDir,
+        filenameTemplate: s.filenameTemplate,
+        sources: s.sources,
+      }),
+    }
+  )
+);

src/store/sidebarStore.ts

@@ -20,6 +20,7 @@ export const DEFAULT_SIDEBAR_ITEMS: SidebarItemConfig[] = [
   { id: 'mostPlayed',    visible: true },
   { id: 'radio',         visible: true },
   { id: 'folderBrowser', visible: false },
+  { id: 'deviceSync',    visible: false },
   { id: 'statistics',    visible: true },
   { id: 'help',          visible: true },
 ];