feat: add session-level API cache — skip spinners on home/albums/artists/playlists revisit

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/api.js

@@ -15,6 +15,27 @@ export async function api(method, extra='') {
   if(r.status !== 'ok') throw new Error(r.error?.message || 'API Error');
   return r;
 }
+
+// ─── API CACHE ───
+const _apiCache = new Map(); // key → { data, ts }
+const _CACHE_TTL = 5 * 60 * 1000; // 5 min default
+
+export function apiIsCached(method, extra = '') {
+  const hit = _apiCache.get(method + extra);
+  return !!(hit && Date.now() - hit.ts < _CACHE_TTL);
+}
+export async function apiCached(method, extra = '', ttl = _CACHE_TTL) {
+  const key = method + extra;
+  const hit = _apiCache.get(key);
+  if (hit && Date.now() - hit.ts < ttl) return hit.data;
+  const data = await api(method, extra);
+  _apiCache.set(key, { data, ts: Date.now() });
+  return data;
+}
+export function invalidateApiCache(method, extra) {
+  if (method) _apiCache.delete(method + (extra || ''));
+  else _apiCache.clear();
+}
 export function streamUrl(id) { return apiUrl('stream', `&id=${id}&format=raw`); }
 
 // Stable per-session cache so the same id+size always produces the same URL.