feat(device-sync): fixed cross-OS naming scheme + playlist folders

Replace the user-configurable filename template with a fixed scheme so
syncing the same library across Linux/Windows/macOS no longer re-downloads
files just because the template string differs by OS.

Track layout on the device:
  {AlbumArtist}/{Album}/{TrackNum:02d} - {Title}.{ext}
  Playlists/{PlaylistName}/{Index:02d} - {Artist} - {Title}.{ext}
  Playlists/{PlaylistName}/{PlaylistName}.m3u8

Playlist tracks go into a self-contained folder with a sibling-referencing
Extended M3U, instead of being scattered across the album tree with an
external .m3u8 — the stick stays tidy even for 50-artist playlists.

Rust (lib.rs):
- TrackSyncInfo: drop disc_number/year (not used in fixed schema), add
  album_artist, duration, playlist_name, playlist_index
- sanitize_path_component strengthens empty-result handling via new
  sanitize_or() which falls back to "Unknown Artist/Album/Title"
- build_track_path() dispatches on playlist context — playlist tracks
  get the "Playlists/{Name}/…" shape, album tracks the traditional tree
- albumArtist resolution: the server's albumArtist tag, falling back to
  the track artist (empty-string treated as missing)
- calculate_sync_payload: dedup key is now (source_id, track_id) instead
  of track_id alone, so a track appearing in both an album and a playlist
  ends up on the device in both locations; playlist context is embedded
  into the response JSON as _playlistName / _playlistIndex so the follow-
  up sync_batch_to_device call can route each track correctly
- write_playlist_m3u8: rewritten — writes into the playlist folder with
  sibling filenames instead of a top-level .m3u8 with ../relative paths
- New rename_device_files command: atomic renames (per-entry result,
  skip-if-exists, skip-if-source-missing) plus empty-dir cleanup. Used
  by the migration flow
- write_device_manifest: v2 format, drops filenameTemplate field
- Drop the `template` parameter from sync_track_to_device,
  compute_sync_paths, calculate_sync_payload, sync_batch_to_device

Frontend:
- DeviceSyncStore: drop filenameTemplate from state and persist
- DeviceSync.tsx: strip template editor UI (presets, token buttons, live
  preview); replace with a compact read-only schema info card
- trackToSyncInfo(track, url, playlistCtx?): optional playlist context
  parameter. Tracks coming back from calculate_sync_payload with embedded
  _playlistName/_playlistIndex fall through via the same parameter
- compute_sync_paths / write_playlist_m3u8 / delete paths now pass the
  playlist context per-source so status checks + deletions hit the right
  files (album tree vs playlist folder)
- Album-Artist fallback: frontend falls back to track artist when the
  server has no albumArtist tag (empty strings treated as missing) —
  "Unknown Artist" is only used as a last-resort placeholder
- New migration flow: "Reorganize existing files…" button opens a modal
  that reads the legacy filenameTemplate from the v1 manifest, computes
  per-track (old, new) rename pairs, detects collisions (two old files
  mapping onto one new path), and executes via rename_device_files.
  Playlist-only tracks are left for the next sync to re-download into
  the new playlist folder
- Small JS-side applyLegacyTemplate() helper: only used by the migration
  preview so we can diff old paths against the current files on disk
- CSS: device-sync-schema-section / -code / -hint, migrate-modal +
  summary + warning + result layout

i18n (en + de):
- Remove dead keys: filenameTemplate, templatePreview, templateHint,
  templatePresetStandard/MultiDisc/AltFolder, tokenSlashHint
- Add schemaLabel, schemaHint, migrateButton/Title/Tooltip/Loading/
  NothingToDo/NoTemplate/FilesToRename/Unchanged/Collisions/
  PreviewNote/Executing/Success/Failed/ShowErrors/Start

Other locales (fr/nl/zh/nb/ru/es) fall through to the English defaultValue
until translated in a follow-up pass.

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

src/store/deviceSyncStore.ts

@@ -9,7 +9,6 @@ export interface DeviceSyncSource {
 
 interface DeviceSyncState {
   targetDir: string | null;
-  filenameTemplate: string;
   sources: DeviceSyncSource[];        // persistent device content list
   checkedIds: string[];               // currently checked for bulk actions (not persisted)
   pendingDeletion: string[];          // source IDs marked for deletion (not persisted)
@@ -17,7 +16,6 @@ interface DeviceSyncState {
   scanning: boolean;                   // true while scanning the device
 
   setTargetDir: (dir: string | null) => void;
-  setFilenameTemplate: (t: string) => void;
   addSource: (source: DeviceSyncSource) => void;
   removeSource: (id: string) => void;
   clearSources: () => void;
@@ -35,7 +33,6 @@ export const useDeviceSyncStore = create<DeviceSyncState>()(
   persist(
     (set) => ({
       targetDir: null,
-      filenameTemplate: '{artist}/{album}/{track_number} - {title}',
       sources: [],
       checkedIds: [],
       pendingDeletion: [],
@@ -43,7 +40,6 @@ export const useDeviceSyncStore = create<DeviceSyncState>()(
       scanning: false,
 
       setTargetDir: (dir) => set({ targetDir: dir }),
-      setFilenameTemplate: (t) => set({ filenameTemplate: t }),
 
       addSource: (source) =>
         set((s) => ({
@@ -97,7 +93,6 @@ export const useDeviceSyncStore = create<DeviceSyncState>()(
       name: 'psysonic_device_sync',
       partialize: (s) => ({
         targetDir: s.targetDir,
-        filenameTemplate: s.filenameTemplate,
         sources: s.sources,
       }),
     }