fix: eliminate audio distortion and sync animation to audio

BeepGenerator: add 5ms attack / 10ms release envelope so the
waveform ramps in/out cleanly instead of cutting off mid-cycle,
which caused the audible click/distortion.

AudioEngine: replace postDelayed scheduling with a continuous
write loop — each iteration writes the full beep followed by
silence in 10ms chunks. This keeps the AudioTrack stream full
and eliminates underrun noise. onBeep callback fires on the
main thread just before each beep is queued.

PacerService: wire onBeep to increment beepTick (Compose state).

PacerScreen: replace the independent BPM timer with beepTick so
the pulse ring animation is driven by the actual audio callback,
eliminating the drift at slow paces.

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

@@ -5,19 +5,30 @@ import android.media.AudioFormat
 import android.media.AudioTrack
 import android.os.Handler
 import android.os.HandlerThread
+import android.os.Looper
 
 /**
  * Plays 880 Hz beeps at a given BPM using AudioTrack.
  *
  * Audio focus is intentionally never requested so beeps play alongside
  * other apps (e.g. AntennaPod) without ducking or interruption (US-05).
+ *
+ * Each iteration writes a full interval buffer (beep + silence) to the
+ * AudioTrack in one shot, keeping the stream continuous and glitch-free.
+ * The [onBeep] callback fires on the main thread just before each beep.
  */
 class AudioEngine {
 
     companion object {
         private const val BEEP_DURATION_MS = 40
+        private const val SILENCE_CHUNK_MS = 10
     }
 
+    private val mainHandler = Handler(Looper.getMainLooper())
+
+    /** Called on the main thread immediately before each beep is queued. */
+    var onBeep: (() -> Unit)? = null
+
     private var handlerThread: HandlerThread? = null
     private var handler: Handler? = null
     private var audioTrack: AudioTrack? = null
@@ -42,7 +53,6 @@ class AudioEngine {
 
         audioTrack = AudioTrack.Builder()
             .setAudioAttributes(
-                // USAGE_MEDIA with no requestAudioFocus call = plays alongside other audio (US-05)
                 AudioAttributes.Builder()
                     .setUsage(AudioAttributes.USAGE_MEDIA)
                     .setContentType(AudioAttributes.CONTENT_TYPE_MUSIC)
@@ -64,8 +74,7 @@ class AudioEngine {
 
         handlerThread = HandlerThread("PacerAudio").also { it.start() }
         handler = Handler(handlerThread!!.looper)
-
-        scheduleBeep()
+        handler?.post(::audioLoop)
     }
 
     fun stop() {
@@ -88,18 +97,28 @@ class AudioEngine {
         audioTrack?.setVolume(currentVolume)
     }
 
-    private fun scheduleBeep() {
-        handler?.post(object : Runnable {
-            override fun run() {
-                if (!playing) return
-                playBeep()
-                handler?.postDelayed(this, BpmCalculator.intervalMs(currentBpm))
-            }
-        })
-    }
+    private fun audioLoop() {
+        val silenceChunk = ShortArray(BeepGenerator.SAMPLE_RATE * SILENCE_CHUNK_MS / 1000)
 
-    private fun playBeep() {
-        val samples = BeepGenerator.generateBeep(BEEP_DURATION_MS)
-        audioTrack?.write(samples, 0, samples.size)
+        while (playing) {
+            val bpm = currentBpm
+            val intervalMs = BpmCalculator.intervalMs(bpm).toInt()
+
+            // Notify UI before writing so animation fires in sync with audio
+            mainHandler.post { onBeep?.invoke() }
+
+            // Write beep
+            val beep = BeepGenerator.generateBeep(BEEP_DURATION_MS)
+            audioTrack?.write(beep, 0, beep.size)
+
+            // Write silence for the rest of the interval in small chunks so
+            // we can respond to stop() quickly
+            var silenceRemaining = BeepGenerator.SAMPLE_RATE * (intervalMs - BEEP_DURATION_MS) / 1000
+            while (silenceRemaining > 0 && playing) {
+                val toWrite = minOf(silenceChunk.size, silenceRemaining)
+                audioTrack?.write(silenceChunk, 0, toWrite)
+                silenceRemaining -= toWrite
+            }
+        }
     }
 }

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

@@ -8,12 +8,22 @@ object BeepGenerator {
     const val SAMPLE_RATE = 44100
     const val FREQUENCY = 880.0
 
+    private const val ATTACK_MS = 5
+    private const val RELEASE_MS = 10
+
     fun generateBeep(durationMs: Int): ShortArray {
         val numSamples = SAMPLE_RATE * durationMs / 1000
+        val attackSamples = SAMPLE_RATE * ATTACK_MS / 1000
+        val releaseSamples = SAMPLE_RATE * RELEASE_MS / 1000
         val samples = ShortArray(numSamples)
         for (i in 0 until numSamples) {
             val angle = 2.0 * PI * FREQUENCY * i / SAMPLE_RATE
-            samples[i] = (sin(angle) * Short.MAX_VALUE).toInt().toShort()
+            val envelope = when {
+                i < attackSamples -> i.toDouble() / attackSamples
+                i >= numSamples - releaseSamples -> (numSamples - i).toDouble() / releaseSamples
+                else -> 1.0
+            }
+            samples[i] = (sin(angle) * Short.MAX_VALUE * envelope).toInt().toShort()
         }
         return samples
     }

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

@@ -10,6 +10,9 @@ import android.content.pm.ServiceInfo
 import android.os.Build
 import android.os.IBinder
 import android.os.PowerManager
+import androidx.compose.runtime.getValue
+import androidx.compose.runtime.mutableLongStateOf
+import androidx.compose.runtime.setValue
 import androidx.core.app.NotificationCompat
 
 class PacerService : Service() {
@@ -28,6 +31,10 @@ class PacerService : Service() {
         var isRunning = false
             private set
 
+        // Incremented on the main thread before each beep — observed by PacerScreen
+        var beepTick by mutableLongStateOf(0L)
+            private set
+
         fun startIntent(context: Context, bpm: Int, volume: Float) =
             Intent(context, PacerService::class.java).apply {
                 action = ACTION_START
@@ -87,6 +94,7 @@ class PacerService : Service() {
             startForeground(NOTIFICATION_ID, notification)
         }
         acquireWakeLock()
+        audioEngine.onBeep = { beepTick++ }
         audioEngine.start(bpm, volume)
     }
 

app/src/main/kotlin/net/jeena/pacer/ui/PacerScreen.kt

@@ -35,7 +35,7 @@ import androidx.compose.ui.text.font.FontWeight
 import androidx.compose.ui.unit.dp
 import androidx.compose.ui.unit.sp
 import kotlinx.coroutines.delay
-import net.jeena.pacer.BpmCalculator
+import net.jeena.pacer.PacerService
 import net.jeena.pacer.PacerViewModel
 import net.jeena.pacer.ui.theme.Green80
 
@@ -56,19 +56,18 @@ fun PacerScreen(viewModel: PacerViewModel, modifier: Modifier = Modifier) {
     val bpm = viewModel.bpm
     val volume = viewModel.volume
 
-    // Pulse trigger: fires once per beat when playing
+    // Driven by the actual audio callback — guaranteed in sync with the beep
+    val beepTick = PacerService.beepTick
     var pulseTrigger by remember { mutableStateOf(false) }
-    LaunchedEffect(isPlaying, bpm) {
-        if (isPlaying) {
-            while (true) {
+    LaunchedEffect(beepTick) {
+        if (beepTick > 0L) {
             pulseTrigger = true
             delay(80)
             pulseTrigger = false
-                delay(BpmCalculator.intervalMs(bpm) - 80)
         }
-        } else {
-            pulseTrigger = false
     }
+    LaunchedEffect(isPlaying) {
+        if (!isPlaying) pulseTrigger = false
     }
 
     val pulseScale by animateFloatAsState(