feat(metadata): implement single-track metadata editor page (§A7)

Replace the placeholder with a controlled form for title/artist/album/
year/genre/track number, an AcoustID "find matches" action showing
ranked candidates with confidence, a diff/apply picker, a re-enrich
button, and save via PUT /metadata. Adds matches/apply API endpoints,
mappers, types, and en/ru i18n strings. Batch editor remains a
placeholder (deferred).

src/i18n/locales/en.ts

@@ -281,6 +281,39 @@ const en = {
       manual: 'Edited manually — not auto-updated',
     },
   },
+  metadataEditor: {
+    error: 'Failed to load track',
+    saved: 'Metadata saved.',
+    saveError: 'Failed to save metadata.',
+    save: 'Save',
+    fields: {
+      title: 'Title',
+      artist: 'Artist',
+      album: 'Album',
+      year: 'Year',
+      genre: 'Genre',
+      trackNumber: 'Track number',
+    },
+    autoEnrich: {
+      title: 'AcoustID lookup',
+      hint: 'Identify this track by audio fingerprint.',
+      findMatches: 'Find matches',
+      reEnrich: 'Re-run enrichment',
+      enqueued: 'Enrichment queued — refresh in a moment.',
+      error: 'Could not look up matches.',
+      noMatches: 'No matches found.',
+    },
+    matches: {
+      use: 'Use',
+      unknownTitle: 'Unknown title',
+    },
+    diff: {
+      title: 'Apply this match?',
+      noChanges: 'No changes from current values.',
+      cancel: 'Cancel',
+      apply: 'Apply',
+    },
+  },
 } as const;
 
 export default en;