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/tracks.ts

@@ -0,0 +1,15 @@
+export const tracks = {
+  title: 'Tracks',
+  subtitle: 'Browse. Search. Discover.',
+  heroEyebrow: 'Track of the moment',
+  heroReroll: 'Pick another',
+  playSong: 'Play',
+  enqueueSong: 'Add to queue',
+  railRandom: 'Random Pick',
+  railHighlyRated: 'Highly Rated',
+  browseTitle: 'Browse all tracks',
+  browseUnsupported: "This server doesn't list the whole library at once. Use the search above to find specific tracks.",
+  searchPlaceholder: 'Find a track by title, artist or album…',
+  count_one: '{{count}} track',
+  count_other: '{{count}} tracks',
+};