feat(US-08, US-09, US-10): implement foreground service with notification

PacerService owns AudioEngine and a PARTIAL_WAKE_LOCK so audio
survives screen lock and Doze during long walks. Foreground
notification shows current BPM and a Stop action button.

PacerViewModel becomes AndroidViewModel and communicates with
the service via intents, keeping AudioEngine off the UI process.
Service uses START_NOT_STICKY so it does not restart if killed.

app/src/main/kotlin/net/jeena/pacer/PacerService.kt

@@ -1,10 +1,153 @@
 package net.jeena.pacer
 
+import android.app.NotificationChannel
+import android.app.NotificationManager
+import android.app.PendingIntent
 import android.app.Service
+import android.content.Context
 import android.content.Intent
+import android.content.pm.ServiceInfo
+import android.os.Build
 import android.os.IBinder
+import android.os.PowerManager
+import androidx.core.app.NotificationCompat
 
-// Stub — full implementation in Theme 4 (US-08)
 class PacerService : Service() {
+
+    companion object {
+        const val ACTION_START = "net.jeena.pacer.START"
+        const val ACTION_STOP = "net.jeena.pacer.STOP"
+        const val ACTION_UPDATE_BPM = "net.jeena.pacer.UPDATE_BPM"
+        const val ACTION_UPDATE_VOLUME = "net.jeena.pacer.UPDATE_VOLUME"
+        const val EXTRA_BPM = "bpm"
+        const val EXTRA_VOLUME = "volume"
+        private const val NOTIFICATION_ID = 1
+        private const val CHANNEL_ID = "pacer"
+
+        // Allows ViewModel to check running state without binding
+        var isRunning = false
+            private set
+
+        fun startIntent(context: Context, bpm: Int, volume: Float) =
+            Intent(context, PacerService::class.java).apply {
+                action = ACTION_START
+                putExtra(EXTRA_BPM, bpm)
+                putExtra(EXTRA_VOLUME, volume)
+            }
+
+        fun stopIntent(context: Context) =
+            Intent(context, PacerService::class.java).apply { action = ACTION_STOP }
+
+        fun updateBpmIntent(context: Context, bpm: Int) =
+            Intent(context, PacerService::class.java).apply {
+                action = ACTION_UPDATE_BPM
+                putExtra(EXTRA_BPM, bpm)
+            }
+
+        fun updateVolumeIntent(context: Context, volume: Float) =
+            Intent(context, PacerService::class.java).apply {
+                action = ACTION_UPDATE_VOLUME
+                putExtra(EXTRA_VOLUME, volume)
+            }
+    }
+
+    private val audioEngine = AudioEngine()
+    private var wakeLock: PowerManager.WakeLock? = null
+    private var currentBpm = 120
+
     override fun onBind(intent: Intent?): IBinder? = null
+
+    override fun onStartCommand(intent: Intent?, flags: Int, startId: Int): Int {
+        when (intent?.action) {
+            ACTION_START -> {
+                currentBpm = intent.getIntExtra(EXTRA_BPM, 120)
+                val volume = intent.getFloatExtra(EXTRA_VOLUME, 1f)
+                startPacing(currentBpm, volume)
+            }
+            ACTION_STOP -> stopPacing()
+            ACTION_UPDATE_BPM -> {
+                currentBpm = intent.getIntExtra(EXTRA_BPM, 120)
+                audioEngine.setBpm(currentBpm)
+                updateNotification(currentBpm)
+            }
+            ACTION_UPDATE_VOLUME -> {
+                audioEngine.setVolume(intent.getFloatExtra(EXTRA_VOLUME, 1f))
+            }
+        }
+        return START_NOT_STICKY
+    }
+
+    private fun startPacing(bpm: Int, volume: Float) {
+        isRunning = true
+        createNotificationChannel()
+        val notification = buildNotification(bpm)
+        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.Q) {
+            startForeground(NOTIFICATION_ID, notification, ServiceInfo.FOREGROUND_SERVICE_TYPE_MEDIA_PLAYBACK)
+        } else {
+            startForeground(NOTIFICATION_ID, notification)
+        }
+        acquireWakeLock()
+        audioEngine.start(bpm, volume)
+    }
+
+    private fun stopPacing() {
+        audioEngine.stop()
+        releaseWakeLock()
+        isRunning = false
+        stopForeground(STOP_FOREGROUND_REMOVE)
+        stopSelf()
+    }
+
+    private fun acquireWakeLock() {
+        val pm = getSystemService(Context.POWER_SERVICE) as PowerManager
+        wakeLock = pm.newWakeLock(PowerManager.PARTIAL_WAKE_LOCK, "Pacer::AudioWakeLock")
+        wakeLock?.acquire()
+    }
+
+    private fun releaseWakeLock() {
+        if (wakeLock?.isHeld == true) wakeLock?.release()
+        wakeLock = null
+    }
+
+    private fun createNotificationChannel() {
+        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
+            val channel = NotificationChannel(
+                CHANNEL_ID,
+                "Pacer",
+                NotificationManager.IMPORTANCE_LOW
+            ).apply { description = "Pacer pacing audio" }
+            getSystemService(NotificationManager::class.java).createNotificationChannel(channel)
+        }
+    }
+
+    private fun buildNotification(bpm: Int): android.app.Notification {
+        val stopPendingIntent = PendingIntent.getService(
+            this, 0, stopIntent(this),
+            PendingIntent.FLAG_UPDATE_CURRENT or PendingIntent.FLAG_IMMUTABLE
+        )
+        val openAppPendingIntent = PendingIntent.getActivity(
+            this, 0,
+            Intent(this, MainActivity::class.java),
+            PendingIntent.FLAG_UPDATE_CURRENT or PendingIntent.FLAG_IMMUTABLE
+        )
+        return NotificationCompat.Builder(this, CHANNEL_ID)
+            .setSmallIcon(R.drawable.ic_notification)
+            .setContentTitle("Pacer")
+            .setContentText("Pacing at $bpm BPM")
+            .setContentIntent(openAppPendingIntent)
+            .addAction(android.R.drawable.ic_media_pause, "Stop", stopPendingIntent)
+            .setOngoing(true)
+            .setSilent(true)
+            .build()
+    }
+
+    private fun updateNotification(bpm: Int) {
+        val nm = getSystemService(NotificationManager::class.java)
+        nm.notify(NOTIFICATION_ID, buildNotification(bpm))
+    }
+
+    override fun onDestroy() {
+        super.onDestroy()
+        stopPacing()
+    }
 }

app/src/main/kotlin/net/jeena/pacer/PacerViewModel.kt

@@ -1,15 +1,16 @@
 package net.jeena.pacer
 
+import android.app.Application
 import androidx.compose.runtime.getValue
 import androidx.compose.runtime.mutableFloatStateOf
 import androidx.compose.runtime.mutableIntStateOf
 import androidx.compose.runtime.mutableStateOf
 import androidx.compose.runtime.setValue
-import androidx.lifecycle.ViewModel
+import androidx.lifecycle.AndroidViewModel
 
-class PacerViewModel : ViewModel() {
+class PacerViewModel(application: Application) : AndroidViewModel(application) {
 
-    private val audioEngine = AudioEngine()
+    private val ctx = application.applicationContext
 
     var bpm by mutableIntStateOf(120)
         private set
@@ -17,30 +18,26 @@ class PacerViewModel : ViewModel() {
     var volume by mutableFloatStateOf(1f)
         private set
 
-    var isPlaying by mutableStateOf(false)
+    var isPlaying by mutableStateOf(PacerService.isRunning)
         private set
 
     fun togglePlayback() {
         if (isPlaying) {
-            audioEngine.stop()
+            ctx.startService(PacerService.stopIntent(ctx))
             isPlaying = false
         } else {
-            audioEngine.start(bpm, volume)
+            ctx.startService(PacerService.startIntent(ctx, bpm, volume))
             isPlaying = true
         }
     }
 
     fun updateBpm(newBpm: Int) {
         bpm = newBpm
-        audioEngine.setBpm(newBpm)
+        if (isPlaying) ctx.startService(PacerService.updateBpmIntent(ctx, newBpm))
     }
 
     fun updateVolume(newVolume: Float) {
         volume = newVolume
-        audioEngine.setVolume(newVolume)
+        if (isPlaying) ctx.startService(PacerService.updateVolumeIntent(ctx, newVolume))
-    }
-
-    override fun onCleared() {
-        audioEngine.stop()
     }
 }

app/src/main/res/drawable/ic_notification.xml

@@ -0,0 +1,11 @@
+<?xml version="1.0" encoding="utf-8"?>
+<vector xmlns:android="http://schemas.android.com/apk/res/android"
+    android:width="24dp"
+    android:height="24dp"
+    android:viewportWidth="24"
+    android:viewportHeight="24">
+    <!-- Simple filled circle — white silhouette required for notification icons -->
+    <path
+        android:fillColor="#FFFFFF"
+        android:pathData="M12,2C6.48,2 2,6.48 2,12s4.48,10 10,10 10,-4.48 10,-10S17.52,2 12,2z" />
+</vector>