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/lib/format.ts

@@ -26,6 +26,20 @@ export function formatDateTime(iso: string | undefined): string | undefined {
   }).format(d);
 }
 
+/** Human "X days Y hours" style for big spans (e.g. total library playtime).
+ * Shows the two most-significant non-zero units; falls back to "0m". */
+export function formatLongDuration(seconds: number): string {
+  if (seconds <= 0) return '0m';
+  const days = Math.floor(seconds / 86400);
+  const hours = Math.floor((seconds % 86400) / 3600);
+  const mins = Math.floor((seconds % 3600) / 60);
+  const parts: string[] = [];
+  if (days) parts.push(`${days}d`);
+  if (hours) parts.push(`${hours}h`);
+  if (mins && parts.length < 2) parts.push(`${mins}m`);
+  return parts.slice(0, 2).join(' ') || '0m';
+}
+
 export function formatCount(n: number): string {
   if (n < 1000) return String(n);
   if (n < 1_000_000) return `${(n / 1000).toFixed(1)}K`;