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/locales/de.ts

@@ -1144,9 +1144,24 @@ export const deTranslation = {
     free: 'frei',
     notMountedVolume: 'Ziel befindet sich nicht auf einem eingehängten Laufwerk. Das Gerät wurde möglicherweise getrennt.',
     chooseFolder: 'Auswählen…',
-    filenameTemplate: 'Dateinamen-Vorlage',
     targetDevice: 'Zielgerät',
-    templateHint: 'Variablen: {artist}, {album}, {title}, {track_number}, {disc_number}, {year}',
+    schemaLabel: 'Benennungsschema',
+    schemaHint: 'Festes Schema für zuverlässige Synchronisation über verschiedene Betriebssysteme hinweg. Playlists werden als .m3u8-Dateien geschrieben und referenzieren die Album-Tracks — keine Duplikate auf dem Gerät.',
+    migrateButton: 'Bestehende Dateien re-organisieren…',
+    migrateTooltip: 'Vorhandene Dateien auf dem Gerät nach dem neuen Schema umbenennen (basierend auf der alten Namensvorlage).',
+    migrateTitle: 'Bestehende Dateien re-organisieren',
+    migrateLoading: 'Analysiere vorhandene Dateien…',
+    migrateNothingToDo: 'Alle vorhandenen Dateien entsprechen bereits dem neuen Schema — nichts zu tun.',
+    migrateNoTemplate: 'Keine alte Namensvorlage auf dem Gerät gefunden. Migration ist nur möglich, wenn der Stick mit einer Psysonic-Version synchronisiert wurde, die konfigurierbare Vorlagen unterstützte.',
+    migrateFilesToRename: 'Dateien werden umbenannt',
+    migrateUnchanged: '{{n}} Dateien liegen bereits am korrekten Pfad',
+    migrateCollisions: '{{n}} Dateien können nicht automatisch umbenannt werden (mehrere Tracks zeigen auf dasselbe Ziel). Sie bleiben unverändert — beim nächsten Sync werden sie neu heruntergeladen.',
+    migratePreviewNote: 'Alte Vorlage: {{tpl}}',
+    migrateExecuting: 'Dateien werden umbenannt…',
+    migrateSuccess: '{{n}} Dateien erfolgreich umbenannt',
+    migrateFailed: '{{n}} Umbenennungen fehlgeschlagen',
+    migrateShowErrors: 'Fehler anzeigen',
+    migrateStart: 'Umbenennen starten',
     onDevice: 'Auf dem Gerät',
     addSources: 'Hinzufügen…',
     colName: 'Name',
@@ -1167,11 +1182,6 @@ export const deTranslation = {
     actionTransfer: 'Auf Gerät übertragen',
     actionDelete: 'Vom Gerät löschen',
     actionApplyAll: 'Änderungen synchronisieren',
-    templatePreview: 'Vorschau',
-    templatePresetStandard: 'Standard',
-    templatePresetMultiDisc: 'Multi-Disc',
-    templatePresetAltFolder: 'Alt. Ordner',
-    tokenSlashHint: '/ = Ordner-Trennzeichen',
     cancel: 'Abbrechen',
     noTargetDir: 'Bitte zuerst einen Zielordner auswählen.',
     noSources: 'Bitte mindestens eine Quelle auswählen.',