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/endpoints/storage.ts

@@ -1,17 +1,16 @@
 import { api } from '../index';
+import { toStorageStats, type RawStorageStats } from '../mappers';
 import type { StorageStats } from '../types';
 
-// NOTE: the backend `/storage` routes are still unimplemented stubs (no body /
-// no schema), and the real paths differ from these placeholders (`GET /storage`,
-// `/storage/duplicates`, `/storage/broken`, `/storage/missing-metadata`,
-// `POST /storage/cleanup`). Re-point paths and add snake→camel mappers (see
-// `mappers.ts`) once the backend defines the storage response shapes; until then
-// these are provisional and unused by the UI.
+// `GET /storage` returns library + disk statistics (§A6). The maintenance
+// routes (`/storage/duplicates`, `/storage/broken`, `/storage/missing-metadata`,
+// `POST /storage/cleanup`) are still backend stubs and unused by the UI.
 
 export const storageApi = api.injectEndpoints({
   endpoints: (build) => ({
     getStorageStats: build.query<StorageStats, void>({
-      query: () => '/storage/stats',
+      query: () => '/storage',
+      transformResponse: (raw: RawStorageStats) => toStorageStats(raw),
       providesTags: ['Storage'],
     }),
     scanStorage: build.mutation<{ jobId: string }, void>({