refactor(i18n): split locale files into per-namespace modules (Phase K) (#688)

Each src/locales/<lang>.ts (~1800 LOC) becomes a folder src/locales/<lang>/
with one module per i18n namespace (44 each) plus an index.ts barrel that
reassembles <lang>Translation in the original key order.

Mechanical, script-driven split with a JSON round-trip check: every
locale object is byte-identical to its pre-split form. i18n.ts is
unchanged — './locales/<lang>' now resolves to the folder index.

The per-namespace settings.ts files land ~440-460 LOC; a single i18n
namespace is the natural, non-arbitrary split unit for a flat string
table, so they are intentionally left whole.

src/locales/de/sidebar.ts

@@ -0,0 +1,37 @@
+export const sidebar = {
+  library: 'Bibliothek',
+  mainstage: 'Mainstage',
+  newReleases: 'Neueste',
+  allAlbums: 'Alle Alben',
+  randomAlbums: 'Zufallsalben',
+  randomPicker: 'Mix erstellen',
+  artists: 'Künstler',
+  composers: 'Komponist*innen',
+  randomMix: 'Zufallsmix',
+  favorites: 'Favoriten',
+  nowPlaying: 'Now Playing',
+  system: 'System',
+  statistics: 'Statistiken',
+  settings: 'Einstellungen',
+  help: 'Hilfe',
+  expand: 'Sidebar einblenden',
+  collapse: 'Sidebar ausblenden',
+  downloadingTracks: '{{n}} Tracks werden gecacht…',
+  syncingTracks: 'Synchronisiere {{done}}/{{total}}…',
+  cancelDownload: 'Download abbrechen',
+  offlineLibrary: 'Offline-Bibliothek',
+  genres: 'Genres',
+  tracks: 'Titel',
+  playlists: 'Playlists',
+  mostPlayed: 'Meistgehört',
+  losslessAlbums: 'Lossless',
+  radio: 'Internetradio',
+  folderBrowser: 'Ordner-Browser',
+  deviceSync: 'Gerätesync',
+  libraryScope: 'Bibliotheksumfang',
+  allLibraries: 'Alle Bibliotheken',
+  expandPlaylists: 'Playlists ausklappen',
+  collapsePlaylists: 'Playlists einklappen',
+  more: 'Mehr',
+  feelingLucky: 'Glücks-Mix',
+};