fix: cover art caching, skip desync, notification tap opens now playing

- api.js: cache coverUrl() per id+size so WebView HTTP cache can work;
  previously each call generated a new random salt/token → unique URL →
  no cache hits → images re-fetched on every render

- player.js: replace _nsBridge('next')/'prev' with explicit seekToIndex
  in nextSong/prevSong so native always jumps to the exact same index JS
  computed; 'next'/'prev' relied on native's internal pointer being in sync
  which caused 'UI shows new song but audio plays old one' when they drifted

- MusicService: set setSessionActivity PendingIntent on MediaSession so
  tapping the playback notification opens the app

- MainActivity: handle open_now_playing extra in onCreate (cold start)
  and onNewIntent (app already running) to show the Now Playing screen

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

src/api.js

@@ -16,6 +16,11 @@ export async function api(method, extra='') {
   return r;
 }
 export function streamUrl(id) { return apiUrl('stream', `&id=${id}&format=raw`); }
+
+// Stable per-session cache so the same id+size always produces the same URL.
+// Consistent URLs let WebView's HTTP cache work — without this every render
+// generates a new token (random salt) and the image is re-fetched every time.
+const _coverUrlCache = new Map();
 export async function getLyricsBySongId(songId) {
   try {
     const r = await api('getLyricsBySongId', `&id=${songId}`);
@@ -23,7 +28,12 @@ export async function getLyricsBySongId(songId) {
   } catch { return []; }
 }
 export function downloadUrl(id) { return apiUrl('download', `&id=${id}`); }
-export function coverUrl(id, size=300) { return id ? apiUrl('getCoverArt', `&id=${id}&size=${size}`) : null; }
+export function coverUrl(id, size=300) {
+  if (!id) return null;
+  const key = `${id}:${size}`;
+  if (!_coverUrlCache.has(key)) _coverUrlCache.set(key, apiUrl('getCoverArt', `&id=${id}&size=${size}`));
+  return _coverUrlCache.get(key);
+}
 
 // ─── NAVIDROME REST AUTH ───
 export async function ndAuth() {