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/styles/components.css

@@ -8035,6 +8035,116 @@ html.no-compositing .fsr-lyric-line.fsrl-active .fsr-lyric-word.active {
   max-width: 600px;
 }
 
+.device-sync-schema-section {
+  display: flex;
+  flex-direction: column;
+  gap: 6px;
+  flex: 1;
+  max-width: 600px;
+}
+.device-sync-schema-code {
+  display: inline-block;
+  background: var(--bg-glass);
+  border: 1px solid var(--border-subtle);
+  border-radius: var(--radius-sm);
+  padding: 6px 10px;
+  font-size: 12px;
+  color: var(--accent);
+  font-family: monospace;
+  user-select: all;
+  align-self: flex-start;
+}
+.device-sync-schema-hint {
+  font-size: 11px;
+  color: var(--text-muted);
+  line-height: 1.5;
+  max-width: 520px;
+}
+.device-sync-migrate-btn {
+  align-self: flex-start;
+  font-size: 11px;
+  padding: 4px 10px;
+  margin-top: 2px;
+}
+
+/* ── Migration modal ──────────────────────────────────────────────────── */
+.device-sync-migrate-modal {
+  min-width: 440px;
+  max-width: 560px;
+}
+.device-sync-migrate-body {
+  display: flex;
+  flex-direction: column;
+  gap: 12px;
+  margin: 12px 0;
+}
+.device-sync-migrate-loading {
+  display: flex;
+  align-items: center;
+  gap: 10px;
+  padding: 16px 4px;
+  color: var(--text-secondary);
+  font-size: 13px;
+}
+.device-sync-migrate-nothing {
+  padding: 12px 4px;
+  color: var(--text-secondary);
+  font-size: 13px;
+  line-height: 1.5;
+}
+.device-sync-migrate-summary {
+  display: flex;
+  flex-direction: column;
+  gap: 6px;
+  font-size: 13px;
+}
+.device-sync-migrate-summary .muted { color: var(--text-muted); font-size: 12px; }
+.device-sync-migrate-warning {
+  display: flex;
+  align-items: flex-start;
+  gap: 8px;
+  padding: 8px 10px;
+  background: var(--accent-dim);
+  border: 1px solid var(--border-subtle);
+  border-radius: var(--radius-sm);
+  color: var(--warning, var(--accent));
+  font-size: 12px;
+  line-height: 1.4;
+}
+.device-sync-migrate-warning svg { flex-shrink: 0; margin-top: 2px; }
+.device-sync-migrate-preview-note {
+  font-size: 11px;
+  color: var(--text-muted);
+  font-family: monospace;
+  padding: 6px 10px;
+  background: var(--bg-glass);
+  border: 1px solid var(--border-subtle);
+  border-radius: var(--radius-sm);
+  word-break: break-all;
+}
+.device-sync-migrate-result { display: flex; flex-direction: column; gap: 6px; }
+.device-sync-migrate-result-line {
+  display: flex;
+  align-items: center;
+  gap: 8px;
+  font-size: 13px;
+}
+.device-sync-migrate-result-line .positive { color: var(--positive, var(--accent)); }
+.device-sync-migrate-result-line .danger   { color: var(--danger); }
+.device-sync-migrate-errors {
+  font-size: 11px;
+  color: var(--text-muted);
+  margin-top: 6px;
+}
+.device-sync-migrate-errors summary { cursor: pointer; color: var(--accent); }
+.device-sync-migrate-errors ul { max-height: 180px; overflow-y: auto; padding-left: 18px; margin: 6px 0 0; }
+.device-sync-migrate-footer {
+  display: flex;
+  justify-content: flex-end;
+  gap: 8px;
+  margin-top: 10px;
+}
+
 .device-sync-target-section {
   display: flex;
   flex-direction: column;