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>

android/app/src/main/kotlin/com/novasonic/app/MainActivity.kt

@@ -29,6 +29,7 @@ class MainActivity : AppCompatActivity(), MusicService.StateListener {
     private lateinit var webBridge: WebBridge
     private var controllerFuture: ListenableFuture<MediaController>? = null
     private var mediaController: MediaController? = null
+    private var pendingOpenNowPlaying = false
 
     // ─── Back-press handling ──────────────────────────────────────────────────
 
@@ -73,6 +74,19 @@ class MainActivity : AppCompatActivity(), MusicService.StateListener {
 
         val url = BuildConfig.NOVASONIC_URL.ifEmpty { "https://demo.navidrome.org" }
         webView.loadUrl(url)
+
+        // Notification tap while app was not running — defer until page is loaded.
+        if (intent?.getBooleanExtra("open_now_playing", false) == true) {
+            pendingOpenNowPlaying = true
+        }
+    }
+
+    override fun onNewIntent(intent: Intent) {
+        super.onNewIntent(intent)
+        // Notification tap while app is already running — WebView is ready immediately.
+        if (intent.getBooleanExtra("open_now_playing", false)) {
+            webView.post { webView.evaluateJavascript("if(window.openNowPlaying)window.openNowPlaying()", null) }
+        }
     }
 
     override fun onResume() { super.onResume(); webView.onResume() }
@@ -128,6 +142,10 @@ class MainActivity : AppCompatActivity(), MusicService.StateListener {
                     updateStateSnapshot()
                     callJs("onReady")
                 }
+                if (pendingOpenNowPlaying) {
+                    pendingOpenNowPlaying = false
+                    webView.post { webView.evaluateJavascript("if(window.openNowPlaying)window.openNowPlaying()", null) }
+                }
             }
         }
         webView.webChromeClient = WebChromeClient()

android/app/src/main/kotlin/com/novasonic/app/MusicService.kt

@@ -1,5 +1,6 @@
 package com.novasonic.app
 
+import android.app.PendingIntent
 import android.content.Intent
 import android.os.Handler
 import android.os.Looper
@@ -91,8 +92,19 @@ class MusicService : MediaSessionService() {
             .setWakeMode(C.WAKE_MODE_NETWORK)    // holds WakeLock + WifiLock through Doze
             .build()
 
+        // Tapping the notification opens the app directly to the Now Playing screen.
+        val notifTapIntent = Intent(this, MainActivity::class.java).apply {
+            putExtra("open_now_playing", true)
+            flags = Intent.FLAG_ACTIVITY_SINGLE_TOP
+        }
+        val notifTapPending = PendingIntent.getActivity(
+            this, 0, notifTapIntent,
+            PendingIntent.FLAG_UPDATE_CURRENT or PendingIntent.FLAG_IMMUTABLE
+        )
+
         mediaSession = MediaSession.Builder(this, player)
             .setBitmapLoader(bitmapLoader)        // use cached loader for notification artwork
+            .setSessionActivity(notifTapPending)
             .build()
 
         player.addListener(PlayerListener())