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/api/types.ts

@@ -161,3 +161,26 @@ export interface ApiError {
   message: string;
   code?: string;
 }
+
+/** One AcoustID candidate from `GET /tracks/{id}/metadata/matches` (§A7). */
+export interface MetadataMatch {
+  acoustid: string;
+  /** Confidence 0..1. */
+  score: number;
+  recordingMbid?: string;
+  releaseGroupMbid?: string;
+  title?: string;
+  artist?: string;
+  album?: string;
+  year?: number;
+}
+
+/** Manual edits / an accepted match, sent to `PUT /tracks/{id}/metadata`. */
+export interface MetadataEdit {
+  title?: string;
+  artistName?: string;
+  albumTitle?: string;
+  year?: number;
+  genre?: string;
+  trackNumber?: number;
+}