feat(metrics): battery + thermal-zone readings with dashboard temp chart

Extends MetricsProvider with thermals() returning a ThermalSnapshot
(battery_percent, battery_temp_c, cpu_temp_c — all optional). Each
provider implements it independently:

- AndroidMetricsProvider reads /sys/class/power_supply/battery/{capacity,
  temp} (battery temp is tenths of degC) and walks
  /sys/class/thermal/thermal_zone*, filtering by zone type
  (cpu/soc/tsens/core) so battery and skin sensors don't dominate the
  reading. Rejects nonsense values like INT_MAX from buggy zones.
- PsutilMetricsProvider uses sensors_battery() and
  sensors_temperatures() when present (Linux+laptops); no-ops on
  Windows/macOS where psutil doesn't expose them.
- NullMetricsProvider returns the empty snapshot.

PerformanceResponse gains battery_percent / battery_temp_c / cpu_temp_c.
The metrics-history ring buffer also carries cpu_temp / battery_pct /
battery_temp per sample so the dashboard can graph them over time.

Frontend dashboard (perf-charts.ts) gets a new Temperature chart card,
hidden by default and revealed only after seed/poll confirms the
backend reports cpu_temp_c. Battery temperature shows inline as a
secondary badge. The GPU card now also hides entirely when the backend
reports gpu=null instead of showing an "unavailable" placeholder.
HOST_ONLY_KEYS prevents the System/App/Both toggle from flipping a
non-existent app dataset for temp.

Tests: 6 new for thermals (battery tenths-of-degC parsing, CPU zone
filtering, fallback when sensors absent, INT_MAX rejection); 18 metrics
tests total; full suite 733 passing.

server/tests/test_metrics_provider.py

@@ -13,6 +13,7 @@ from ledgrab.utils.metrics import (
     NullMetricsProvider,
     ProcessSnapshot,
     PsutilMetricsProvider,
+    ThermalSnapshot,
     get_metrics_provider,
     reset_metrics_provider,
 )
@@ -181,3 +182,97 @@ def test_factory_prefers_android_when_running_on_android(monkeypatch) -> None:
     monkeypatch.setattr("ledgrab.utils.metrics._android_supported", lambda: True)
     provider = get_metrics_provider()
     assert isinstance(provider, AndroidMetricsProvider)
+
+
+# ── Thermals ────────────────────────────────────────────────────────
+
+
+def test_null_provider_thermals_are_all_none() -> None:
+    snap = NullMetricsProvider().thermals()
+    assert snap == ThermalSnapshot()
+
+
+def test_psutil_provider_thermals_picks_hottest_sensor() -> None:
+    psutil_mock = MagicMock()
+    psutil_mock.Process.return_value = MagicMock()
+    psutil_mock.cpu_count.return_value = 4
+    bat = MagicMock(percent=78.0)
+    psutil_mock.sensors_battery.return_value = bat
+    psutil_mock.sensors_temperatures.return_value = {
+        "coretemp": [
+            MagicMock(current=55.0),
+            MagicMock(current=72.5),
+        ],
+        "acpi": [MagicMock(current=40.0)],
+    }
+    provider = PsutilMetricsProvider(psutil_mock)
+    snap = provider.thermals()
+    assert snap.battery_percent == 78.0
+    assert snap.cpu_temp_c == 72.5  # hottest across all sensors
+    assert snap.battery_temp_c is None  # psutil doesn't expose battery temp
+
+
+def test_psutil_provider_thermals_handles_missing_sensors() -> None:
+    psutil_mock = MagicMock()
+    psutil_mock.Process.return_value = MagicMock()
+    psutil_mock.cpu_count.return_value = 1
+    # Strip the optional sensor methods entirely (e.g. Windows psutil).
+    del psutil_mock.sensors_battery
+    del psutil_mock.sensors_temperatures
+    provider = PsutilMetricsProvider(psutil_mock)
+    assert provider.thermals() == ThermalSnapshot()
+
+
+def test_android_battery_parses_tenths_of_celsius(monkeypatch) -> None:
+    def _fake_int(path: str):
+        return {
+            "/sys/class/power_supply/battery/capacity": 78,
+            "/sys/class/power_supply/battery/temp": 312,  # tenths of °C → 31.2°C
+        }.get(path)
+
+    monkeypatch.setattr(android_mod, "_read_int_file", _fake_int)
+    pct, temp = android_mod._read_battery()
+    assert pct == 78.0
+    assert temp == 31.2
+
+
+def test_android_cpu_temp_filters_non_cpu_zones_and_picks_hottest(monkeypatch) -> None:
+    monkeypatch.setattr(
+        "glob.glob",
+        lambda _: [
+            "/sys/class/thermal/thermal_zone0",
+            "/sys/class/thermal/thermal_zone1",
+            "/sys/class/thermal/thermal_zone2",
+            "/sys/class/thermal/thermal_zone3",
+        ],
+    )
+
+    def _fake_text(path: str):
+        return {
+            "/sys/class/thermal/thermal_zone0/type": "battery",
+            "/sys/class/thermal/thermal_zone1/type": "cpu-thermal",
+            "/sys/class/thermal/thermal_zone2/type": "soc-max",
+            "/sys/class/thermal/thermal_zone3/type": "skin-therm",
+        }.get(path)
+
+    def _fake_int(path: str):
+        return {
+            # Battery & skin should be filtered out by zone type
+            "/sys/class/thermal/thermal_zone0/temp": 99000,
+            "/sys/class/thermal/thermal_zone1/temp": 52000,  # 52°C
+            "/sys/class/thermal/thermal_zone2/temp": 67500,  # 67.5°C ← hottest
+            "/sys/class/thermal/thermal_zone3/temp": 99000,
+        }.get(path)
+
+    monkeypatch.setattr(android_mod, "_read_text_file", _fake_text)
+    monkeypatch.setattr(android_mod, "_read_int_file", _fake_int)
+
+    assert android_mod._read_cpu_temp_c() == 67.5
+
+
+def test_android_cpu_temp_rejects_nonsense_values(monkeypatch) -> None:
+    monkeypatch.setattr("glob.glob", lambda _: ["/sys/class/thermal/thermal_zone0"])
+    monkeypatch.setattr(android_mod, "_read_text_file", lambda _: "cpu-thermal")
+    # Some buggy zones report INT_MAX
+    monkeypatch.setattr(android_mod, "_read_int_file", lambda _: 2147483647)
+    assert android_mod._read_cpu_temp_c() is None