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/en/contextMenu.ts

@@ -0,0 +1,33 @@
+export const contextMenu = {
+  playNow: 'Play Now',
+  playNext: 'Play Next',
+  addToQueue: 'Add to Queue',
+  enqueueAlbum: 'Enqueue Album',
+  enqueueAlbums_one: 'Enqueue {{count}} Album',
+  enqueueAlbums_other: 'Enqueue {{count}} Albums',
+  startRadio: 'Start Radio',
+  instantMix: 'Instant Mix',
+  instantMixFailed: 'Could not build Instant Mix — server or plugin error.',
+  cliMixNeedsTrack: 'Nothing is playing — start playback first, then run the mix command again.',
+  lfmLove: 'Love on Last.fm',
+  lfmUnlove: 'Unlove on Last.fm',
+  favorite: 'Favorite',
+  favoriteArtist: 'Favorite Artist',
+  favoriteAlbum: 'Favorite Album',
+  unfavorite: 'Remove from Favorites',
+  unfavoriteArtist: 'Remove Artist from Favorites',
+  unfavoriteAlbum: 'Remove Album from Favorites',
+  removeFromQueue: 'Remove from Queue',
+  removeFromPlaylist: 'Remove from Playlist',
+  openAlbum: 'Open Album',
+  goToArtist: 'Go to Artist',
+  download: 'Download (ZIP)',
+  addToPlaylist: 'Add to Playlist',
+  selectedPlaylists: '{{count}} playlists selected',
+  selectedAlbums: '{{count}} albums selected',
+  selectedArtists: '{{count}} artists selected',
+  songInfo: 'Song Info',
+  shareLink: 'Copy share link',
+  shareCopied: 'Share link copied to the clipboard.',
+  shareCopyFailed: 'Could not copy to the clipboard.',
+};