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.

frontend/js/gps.js

@@ -840,6 +840,7 @@ GPS._startHrMonitor = async function() {
     GPS.recording.hrPollInterval = setInterval(async () => {
       try {
         const reading = await window.__TAURI__.core.invoke("ble_get_hr");
+        if (reading && reading.battery != null) { GPS.recording.currentBattery = reading.battery; GPS._checkBatteryAlert(reading.battery); }
         if (reading && reading.hr > 0) {
           GPS.recording.currentHr = reading.hr;
           GPS.recording.currentBleData = reading;
@@ -867,6 +868,47 @@ GPS._stopHrMonitor = function() {
   try { window.__TAURI__.core.invoke("ble_clear_hr"); } catch {}
 };
 
+// Low-battery warning for the HR strap: announce (voice) and beep ONCE at 10% and 5%.
+// Resets when the strap reads healthy again (recharged / battery replaced).
+GPS._batAlerted = {};
+GPS._checkBatteryAlert = function(bat) {
+  if (bat == null || bat < 0 || bat > 100) return;
+  if (bat > 15) { GPS._batAlerted = {}; return; }
+  for (let thr of [10, 5]) {
+    if (bat <= thr && !GPS._batAlerted[thr]) {
+      GPS._batAlerted[thr] = true;
+      GPS._speakOrBeep(`Заряд пульсометра ${thr} процентов`);
+    }
+  }
+};
+GPS._speakOrBeep = function(text) {
+  let spoke = false;
+  try {
+    if (window.speechSynthesis && typeof SpeechSynthesisUtterance !== "undefined") {
+      let u = new SpeechSynthesisUtterance(text);
+      u.lang = "ru-RU"; u.rate = 0.95;
+      window.speechSynthesis.speak(u);
+      spoke = true;
+    }
+  } catch {}
+  // Always beep too (TTS may be absent on a de-Googled ROM; the beep is the guaranteed cue).
+  try { GPS._beep(spoke ? 1 : 3); } catch {}
+};
+GPS._beep = function(times) {
+  times = times || 1;
+  let ctx = new (window.AudioContext || window.webkitAudioContext)();
+  let one = (i) => {
+    let o = ctx.createOscillator(), g = ctx.createGain();
+    o.frequency.value = 880; o.type = "sine";
+    o.connect(g); g.connect(ctx.destination);
+    g.gain.value = 0.25;
+    let t0 = ctx.currentTime + i * 0.35;
+    o.start(t0); o.stop(t0 + 0.2);
+  };
+  for (let i = 0; i < times; i++) one(i);
+  setTimeout(() => { try { ctx.close(); } catch {} }, times * 350 + 400);
+};
+
 // Read points from native service file
 // Parse raw JSONL (one native point per line) into sorted point objects.
 GPS._parseNativePoints = function(raw) {

frontend/js/main.js

@@ -4514,6 +4514,10 @@ function renderWorkoutInline(container) {
     if (avgEl) avgEl.textContent = hrs.length ? Math.round(hrs.reduce((a, b) => a + b, 0) / hrs.length) : "--";
     if (maxEl) maxEl.textContent = hrs.length ? Math.max(...hrs) : "--";
     if (GPS.recording.currentHr && hrEl) hrEl.textContent = GPS.recording.currentHr;
+    if (statusEl && GPS.recording.currentHr) {
+      let b = GPS.recording.currentBattery;
+      statusEl.textContent = "Пульсометр на связи" + (b != null ? ` · заряд ${b}%` : "");
+    }
   };
   clearInterval(window._wkTimer);
   window._wkTimer = setInterval(tick, 1000);