Add low-battery warning and auto-shutdown to the tray icon

The oneUpPower kernel module powered the system off at low SOC, but the
icon now reads the battery over I2C and no longer relies on that module.
Restore the safety net in the icon itself.

While running on battery, warn via a tray notification at <=10% and run
`systemctl poweroff` at <=5%. systemctl poweroff is authorised for the
active desktop session without sudo (logind CanPowerOff returns "yes").
State resets when AC is reconnected.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

monitor/batteryIcon.py

@@ -7,10 +7,14 @@ management IC over I2C (bus 1, address 0x64) -- the same registers the
 oneUpPower kernel module uses. This means the icon works for an ordinary
 user (member of the ``i2c`` group) without sudo and without the kernel
 module being loaded. Updates every 5 seconds.
+
+When running on battery it also warns on low charge and powers the system
+off when critically low, mirroring the oneUpPower kernel module.
 """
 
 import sys
 import time
+import subprocess
 
 import smbus2
 from PyQt5.QtWidgets import QApplication, QSystemTrayIcon, QMenu, QAction
@@ -24,6 +28,10 @@ SOC_REG = 0x04          # state of charge, percent
 CURRENT_HIGH_REG = 0x0E  # bit 7 set => running on battery (not plugged in)
 UPDATE_INTERVAL_MS = 5000
 
+# Auto-shutdown while running on battery (mirrors the oneUpPower module).
+SHUTDOWN_SOC = 5   # power off at or below this percentage
+WARN_SOC = 10      # warn the user at or below this percentage
+
 
 def read_battery(bus):
     """Return (percent, plugged_in) read over I2C, or (None, None) on error.
@@ -115,6 +123,8 @@ class BatteryTray:
             time.sleep(2)
 
         self.bus = smbus2.SMBus(I2C_BUS)
+        self.shutdown_triggered = False
+        self.warned = False
         self.tray = QSystemTrayIcon()
 
         # Context menu
@@ -155,6 +165,8 @@ class BatteryTray:
             tooltip = f"Battery: {percent}% ({state})"
             self.tray.setToolTip(tooltip)
             self.status_action.setText(tooltip)
+
+            self.check_power(percent, plugged_in)
         else:
             # I2C read failed - IC not reachable
             icon = make_battery_icon(0, False)
@@ -162,6 +174,35 @@ class BatteryTray:
             self.tray.setToolTip("Battery: N/A (I2C read failed)")
             self.status_action.setText("Battery: N/A (I2C read failed)")
 
+    def check_power(self, percent, plugged_in):
+        """Warn on low battery and power off when critically low.
+
+        Only acts while running on battery. `systemctl poweroff` works for
+        the active desktop session without sudo.
+        """
+        if plugged_in:
+            self.warned = False
+            return
+
+        if percent <= SHUTDOWN_SOC and not self.shutdown_triggered:
+            self.shutdown_triggered = True
+            self.tray.showMessage(
+                "Battery critical",
+                f"Battery at {percent}% - shutting down now.",
+                QSystemTrayIcon.Critical, 10000)
+            try:
+                subprocess.run(["systemctl", "poweroff"],
+                               check=True, timeout=10)
+            except (subprocess.SubprocessError, OSError) as e:
+                print(f"poweroff failed: {e}", file=sys.stderr)
+                self.shutdown_triggered = False  # allow retry next cycle
+        elif percent <= WARN_SOC and not self.warned:
+            self.warned = True
+            self.tray.showMessage(
+                "Battery low",
+                f"Battery at {percent}% - save your work.",
+                QSystemTrayIcon.Warning, 10000)
+
     def run(self):
         return self.app.exec()