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/entityRating.ts

@@ -0,0 +1,9 @@
+export const entityRating = {
+  albumShort: 'Album rating',
+  artistShort: 'Artist rating',
+  albumAriaLabel: 'Album rating',
+  artistAriaLabel: 'Artist rating',
+  selectedArtistsRatingAriaLabel: 'Star rating for {{count}} selected artists',
+  selectedAlbumsRatingAriaLabel: 'Star rating for {{count}} selected albums',
+  saveFailed: 'Could not save rating.',
+};