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/smartPlaylists.ts

@@ -0,0 +1,41 @@
+export const smartPlaylists = {
+  sectionBasic: '1. Basic',
+  sectionGenres: '2. Genres',
+  sectionYearsAndFilters: '3. Years and filters',
+  genreMode: 'Genre mode',
+  genreModeInclude: 'Include genres',
+  genreModeExclude: 'Exclude genres',
+  genreSearchPlaceholder: 'Filter genres...',
+  availableGenres: 'Available',
+  selectedGenres: 'Selected',
+  yearMode: 'Year mode',
+  yearModeInclude: 'Include range',
+  yearModeExclude: 'Exclude range',
+  name: 'Name (without prefix)',
+  limit: 'Limit',
+  limitHint: 'How many tracks to include in playlist (1-{{max}}, usually 50).',
+  artistContains: 'Artist contains…',
+  albumContains: 'Album contains…',
+  titleContains: 'Title contains…',
+  minRating: 'Minimum rating',
+  minRatingAria: 'Minimum rating for smart playlist',
+  minRatingHint: '0 disables minimum threshold; 1-5 keeps tracks with rating above selected value.',
+  fromYear: 'From year',
+  toYear: 'To year',
+  excludeUnrated: 'Exclude unrated tracks',
+  compilationOnly: 'Only compilations',
+  create: 'New Smart Playlist',
+  save: 'Save Smart Playlist',
+  created: 'Created {{name}}',
+  updated: 'Updated {{name}}',
+  createFailed: 'Could not create smart playlist.',
+  updateFailed: 'Could not update smart playlist.',
+  navidromeOnly: 'Smart playlists can only be created on Navidrome servers.',
+  loadFailed: 'Could not load smart playlists from Navidrome.',
+  sortRandom: 'Sort: random',
+  sortTitleAsc: 'Sort: title A-Z',
+  sortTitleDesc: 'Sort: title Z-A',
+  sortYearDesc: 'Sort: year desc',
+  sortYearAsc: 'Sort: year asc',
+  sortPlayCountDesc: 'Sort: play count desc',
+};