feat(storage): functional Storage dashboard (§A6)

Replace the "coming soon" stub with a real dashboard wired to
`GET /storage`. modern-sk visuals: a layered disk-capacity gauge (library
share vs other-used vs free), stat tiles (tracks/artists/albums/playtime/
footprint/avg size), per-format size bars, metadata-health badges, source
breakdown, a popularity-weighted top-genres cloud, and playful fun facts.

- types: full `StorageStats` shape + `toStorageStats` snake→camel mapper
- endpoint: re-point `getStorageStats` to `GET /storage` with transform
- lib: `formatLongDuration` for big playtime spans
- i18n: `storage.*` keys (en + ru)
- three list states (loading / error / empty) per the UI invariant

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/api/types.ts

@@ -96,12 +96,40 @@ export interface UploadResponse {
   already_exists: boolean;
 }
 
-export interface StorageStats {
-  totalBytes: number;
-  usedBytes: number;
+export interface StorageFormatBreakdown {
+  fileFormat: string;
   trackCount: number;
-  albumCount: number;
-  artistCount: number;
+  totalSize: number;
+}
+
+export interface StorageGenreCount {
+  genre: string;
+  trackCount: number;
+}
+
+/** Capacity of the volume backing the media store. Absent for object-store
+ * backends (S3), which have no fixed disk to report. */
+export interface StorageDiskUsage {
+  total: number;
+  used: number;
+  free: number;
+}
+
+export interface StorageStats {
+  totalTracks: number;
+  totalArtists: number;
+  totalAlbums: number;
+  /** Sum of every track's recorded file size (the library's footprint). */
+  totalSize: number;
+  totalDurationSeconds: number;
+  largestTrackSize: number;
+  earliestAdded?: string;
+  latestAdded?: string;
+  byFormat: StorageFormatBreakdown[];
+  byMetadataStatus: Record<string, number>;
+  bySource: Record<string, number>;
+  topGenres: StorageGenreCount[];
+  disk?: StorageDiskUsage;
 }
 
 export interface User {