feat(US-12, US-13): request battery exemption on launch, verify test coverage

MainActivity now requests battery optimization exemption on first
launch so the foreground service survives Doze mode during long
walks. The REQUEST_IGNORE_BATTERY_OPTIMIZATIONS permission was
already declared in the manifest.

Unit test coverage confirmed across all core logic:
BeepGeneratorTest (4), BpmCalculatorTest (6), SettingsRepositoryTest (6).

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

@@ -1,6 +1,10 @@
 package net.jeena.pacer
 
+import android.content.Intent
+import android.net.Uri
 import android.os.Bundle
+import android.os.PowerManager
+import android.provider.Settings
 import androidx.activity.ComponentActivity
 import androidx.activity.compose.setContent
 import androidx.activity.enableEdgeToEdge
@@ -18,6 +22,7 @@ class MainActivity : ComponentActivity() {
 
     override fun onCreate(savedInstanceState: Bundle?) {
         super.onCreate(savedInstanceState)
+        requestBatteryOptimizationExemption()
         enableEdgeToEdge()
         setContent {
             PacerTheme {
@@ -30,4 +35,15 @@ class MainActivity : ComponentActivity() {
             }
         }
     }
+
+    private fun requestBatteryOptimizationExemption() {
+        val pm = getSystemService(PowerManager::class.java)
+        if (!pm.isIgnoringBatteryOptimizations(packageName)) {
+            startActivity(
+                Intent(Settings.ACTION_REQUEST_IGNORE_BATTERY_OPTIMIZATIONS).apply {
+                    data = Uri.parse("package:$packageName")
+                }
+            )
+        }
+    }
 }