HR strap battery + low-charge voice/beep alerts at 10% and 5%

- BleHrService reads the standard Battery service (0x180F/0x2A19) 2.5s after
  the HR subscribe and every 60s; writes 'bat' into ble_hr_data.json.
- HrReading gains battery: Option<u8> (serde alias 'bat').
- gps.js: the HR poll captures reading.battery; _checkBatteryAlert speaks
  (speechSynthesis ru-RU) and beeps once at 10% and 5% (beep is the
  guaranteed cue where TTS is absent), resetting above 15%.
- Workout screen shows the strap charge in its status line.

src-tauri/gen/android/app/src/main/java/dev/baldav/app/BleHrService.kt

@@ -31,6 +31,9 @@ class BleHrService : Service() {
         val HR_SERVICE_UUID: UUID = UUID.fromString("0000180d-0000-1000-8000-00805f9b34fb")
         val HR_MEASUREMENT_UUID: UUID = UUID.fromString("00002a37-0000-1000-8000-00805f9b34fb")
         val CCCD_UUID: UUID = UUID.fromString("00002902-0000-1000-8000-00805f9b34fb")
+        // Standard Battery Service (strap charge)
+        val BATTERY_SERVICE: UUID = UUID.fromString("0000180f-0000-1000-8000-00805f9b34fb")
+        val BATTERY_LEVEL: UUID = UUID.fromString("00002a19-0000-1000-8000-00805f9b34fb")
 
         // FitPro Nordic UART
         val FITPRO_SERVICE: UUID = UUID.fromString("6e400001-b5a3-f393-e0a9-e50e24dcca9d")
@@ -51,6 +54,7 @@ class BleHrService : Service() {
     private var gatt: BluetoothGatt? = null
     private var deviceType = "hr_standard"
     private var dataFile: File? = null
+    private var lastBattery: Int = -1 // strap charge %, -1 = unknown
     private var scanResultsFile: File? = null
     private var fitproWriteChar: BluetoothGattCharacteristic? = null
     private var fitproGatt: BluetoothGatt? = null
@@ -260,6 +264,15 @@ class BleHrService : Service() {
         override fun onCharacteristicChanged(g: BluetoothGatt, char: BluetoothGattCharacteristic) {
             onCharacteristicChanged(g, char, char.value)
         }
+        // Battery level read (0x2A19). Two overloads: API 33+ passes the value, older reads char.value.
+        override fun onCharacteristicRead(g: BluetoothGatt, char: BluetoothGattCharacteristic, value: ByteArray, status: Int) {
+            if (char.uuid == BATTERY_LEVEL && value.isNotEmpty()) { lastBattery = value[0].toInt() and 0xFF; Log.d(TAG, "Battery: $lastBattery%") }
+        }
+        @Deprecated("Deprecated in API 33")
+        override fun onCharacteristicRead(g: BluetoothGatt, char: BluetoothGattCharacteristic, status: Int) {
+            val v = char.value
+            if (char.uuid == BATTERY_LEVEL && v != null && v.isNotEmpty()) { lastBattery = v[0].toInt() and 0xFF; Log.d(TAG, "Battery: $lastBattery%") }
+        }
 
         override fun onDescriptorWrite(g: BluetoothGatt, descriptor: BluetoothGattDescriptor, status: Int) {
             Log.d(TAG, "onDescriptorWrite status=$status")
@@ -284,6 +297,25 @@ class BleHrService : Service() {
             g.writeDescriptor(desc)
         }
         Log.d(TAG, "Standard HR setup complete")
+        // Strap battery (0x180F/0x2A19): read once after the HR subscribe settles, then every 60s.
+        android.os.Handler(mainLooper).also { h ->
+            val readBat = object : Runnable {
+                override fun run() {
+                    if (!isRunning) return
+                    readBattery(g)
+                    h.postDelayed(this, 60000)
+                }
+            }
+            h.postDelayed(readBat, 2500)
+        }
+    }
+
+    private fun readBattery(g: BluetoothGatt) {
+        try {
+            val bs = g.getService(BATTERY_SERVICE) ?: return
+            val bc = bs.getCharacteristic(BATTERY_LEVEL) ?: return
+            g.readCharacteristic(bc)
+        } catch (e: Exception) { Log.e(TAG, "readBattery: ${e.message}") }
     }
 
     private val gattQueue = mutableListOf<() -> Unit>()
@@ -382,8 +414,9 @@ class BleHrService : Service() {
         }
         if (hr <= 0) return
         val ts = System.currentTimeMillis() / 1000
-        writeData("""{"hr":$hr,"ts":$ts}""")
-        Log.d(TAG, "HR: $hr")
+        val batJson = if (lastBattery in 0..100) ""","bat":$lastBattery""" else ""
+        writeData("""{"hr":$hr,"ts":$ts$batJson}""")
+        Log.d(TAG, "HR: $hr bat: $lastBattery")
     }
 
     private fun parseFitProData(data: ByteArray) {

src-tauri/src/ble.rs

@@ -25,6 +25,8 @@ pub struct HrReading {
     pub timestamp: u64,
     #[serde(default)]
     pub device: String,
+    #[serde(alias = "bat", default)]
+    pub battery: Option<u8>,
 }
 
 // Shared state for BLE operations