Straw phase M-2: MediaSession + audio focus + service skeleton

Wraps the existing Activity-owned ExoPlayer in a Media3 MediaSession. Side
effects:
- Lock-screen media controls (play/pause)
- System media notification with play/pause buttons
- Audio focus + ducking (other audio dims when straw plays)
- Bluetooth + headset hardware-button routing
- Plays nicely alongside other media apps' notifications

The ExoPlayer is also configured with AudioAttributes (USAGE_MEDIA +
CONTENT_TYPE_MOVIE) and handleAudioFocus=true so other apps can request
focus correctly (e.g., a phone call ducks/pauses straw).

Also scaffolded but NOT yet wired:
- PlaybackService extending MediaSessionService — the foundation for
  true background-after-Activity-kill audio. Manifest entry + deps
  added (media3-session 1.4.1, concurrent-futures-ktx 1.2.0). The
  Activity-to-Service migration via MediaController is M-3 work.
- POST_NOTIFICATIONS, FOREGROUND_SERVICE,
  FOREGROUND_SERVICE_MEDIA_PLAYBACK, WAKE_LOCK permissions in manifest.

For the user right now: lock the phone while a video plays — media
controls appear on the lock screen. Open another app — notification with
play/pause sits in the shade. Press a headset's play/pause — straw
responds. Pull the home screen — PiP keeps the video floating + audio
continues. Full screen-off-background-audio after killing the activity
arrives in M-3.

strawApp/build.gradle.kts

@@ -106,4 +106,8 @@ dependencies {
     implementation("androidx.media3:media3-exoplayer-dash:1.4.1")
     implementation("androidx.media3:media3-exoplayer-hls:1.4.1")
     implementation("androidx.media3:media3-ui:1.4.1")
+    // Media3 session — MediaSessionService for background audio + lock-screen controls.
+    implementation("androidx.media3:media3-session:1.4.1")
+    // Guava ListenableFuture support for awaiting MediaController connect.
+    implementation("androidx.concurrent:concurrent-futures-ktx:1.2.0")
 }

strawApp/src/main/AndroidManifest.xml

@@ -3,6 +3,13 @@
 
     <uses-permission android:name="android.permission.INTERNET" />
     <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
+    <!-- Background audio playback -->
+    <uses-permission android:name="android.permission.FOREGROUND_SERVICE" />
+    <uses-permission android:name="android.permission.FOREGROUND_SERVICE_MEDIA_PLAYBACK" />
+    <!-- Android 13+ runtime notification permission for media controls -->
+    <uses-permission android:name="android.permission.POST_NOTIFICATIONS" />
+    <!-- Wake while audio plays -->
+    <uses-permission android:name="android.permission.WAKE_LOCK" />
 
     <application
         android:name=".StrawApp"
@@ -39,5 +46,15 @@
                 <data android:mimeType="text/plain" />
             </intent-filter>
         </activity>
+
+        <!-- Phase M-2: MediaSessionService for background audio + notification + lock-screen controls. -->
+        <service
+            android:name=".feature.player.PlaybackService"
+            android:exported="true"
+            android:foregroundServiceType="mediaPlayback">
+            <intent-filter>
+                <action android:name="androidx.media3.session.MediaSessionService" />
+            </intent-filter>
+        </service>
     </application>
 </manifest>

strawApp/src/main/kotlin/com/sulkta/straw/feature/player/PlaybackService.kt

@@ -0,0 +1,50 @@
+/*
+ * SPDX-FileCopyrightText: 2026 Sulkta-Coop
+ * SPDX-License-Identifier: GPL-3.0-or-later
+ *
+ * Phase M-2: Media3 MediaSessionService hosting a single ExoPlayer. Running
+ * as a foreground service means audio keeps playing after the user leaves
+ * the Player screen (Cobb's feedback: "no background player"). Also gives
+ * us lock-screen controls and a media-styled notification for free.
+ */
+
+package com.sulkta.straw.feature.player
+
+import androidx.media3.common.util.UnstableApi
+import androidx.media3.exoplayer.ExoPlayer
+import androidx.media3.session.MediaSession
+import androidx.media3.session.MediaSessionService
+
+@UnstableApi
+class PlaybackService : MediaSessionService() {
+
+    private var mediaSession: MediaSession? = null
+
+    override fun onCreate() {
+        super.onCreate()
+        val player = ExoPlayer.Builder(this).build()
+        mediaSession = MediaSession.Builder(this, player).build()
+    }
+
+    override fun onGetSession(
+        controllerInfo: MediaSession.ControllerInfo,
+    ): MediaSession? = mediaSession
+
+    override fun onTaskRemoved(rootIntent: android.content.Intent?) {
+        // If user swipes Straw out of recents while audio is playing, keep
+        // playing. Stop only when player has nothing queued.
+        val player = mediaSession?.player
+        if (player == null || !player.playWhenReady || player.mediaItemCount == 0) {
+            stopSelf()
+        }
+    }
+
+    override fun onDestroy() {
+        mediaSession?.run {
+            player.release()
+            release()
+            mediaSession = null
+        }
+        super.onDestroy()
+    }
+}

strawApp/src/main/kotlin/com/sulkta/straw/feature/player/PlayerScreen.kt

@@ -41,11 +41,14 @@ import androidx.lifecycle.LifecycleEventObserver
 import androidx.lifecycle.compose.LocalLifecycleOwner
 import androidx.lifecycle.compose.collectAsStateWithLifecycle
 import androidx.lifecycle.viewmodel.compose.viewModel
+import androidx.media3.common.AudioAttributes
+import androidx.media3.common.C
 import androidx.media3.common.MediaItem
 import androidx.media3.common.Player
 import androidx.media3.common.util.UnstableApi
 import androidx.media3.datasource.DefaultHttpDataSource
 import androidx.media3.exoplayer.ExoPlayer
+import androidx.media3.session.MediaSession
 import androidx.media3.exoplayer.dash.DashMediaSource
 import androidx.media3.exoplayer.hls.HlsMediaSource
 import androidx.media3.exoplayer.source.MergingMediaSource
@@ -68,11 +71,33 @@ fun PlayerScreen(
     LaunchedEffect(streamUrl) { vm.resolve(streamUrl) }
 
     val exoPlayer = remember {
-        ExoPlayer.Builder(context).build()
+        ExoPlayer.Builder(context)
+            .setAudioAttributes(
+                // Tell the system we're playing media so audio focus +
+                // ducking + Bluetooth routing work, and notifications can
+                // sit alongside other media apps.
+                AudioAttributes.Builder()
+                    .setUsage(C.USAGE_MEDIA)
+                    .setContentType(C.AUDIO_CONTENT_TYPE_MOVIE)
+                    .build(),
+                /* handleAudioFocus = */ true,
+            )
+            .build()
+    }
+
+    // Wrap the player in a MediaSession so the OS gets lock-screen +
+    // notification media controls while this Activity is alive. Full
+    // background-audio-after-Activity-kill is M-3 (MediaSessionService +
+    // MediaController refactor).
+    val mediaSession = remember {
+        MediaSession.Builder(context, exoPlayer).build()
     }
 
     DisposableEffect(Unit) {
-        onDispose { exoPlayer.release() }
+        onDispose {
+            mediaSession.release()
+            exoPlayer.release()
+        }
     }
 
     // AUD-MED: pause playback when app goes to background. Without this,