Add basic clock support

lib/hal/HalPowerManager.h

@@ -29,9 +29,11 @@ class HalPowerManager {
   // Control CPU frequency for power saving
   void setPowerSaving(bool enabled);
 
-  // Setup wake up GPIO and enter deep sleep
-  // Should be called inside main loop() to handle the currentLockMode
-  void startDeepSleep(HalGPIO& gpio) const;
+  // Setup wake up GPIO and enter deep sleep.
+  // When keepClockAlive is true, GPIO13 stays HIGH so the LP timer keeps
+  // running during sleep (~3-4 mA extra).  This allows HalClock to compute
+  // elapsed sleep time and restore the wall clock accurately on wake.
+  void startDeepSleep(HalGPIO& gpio, bool keepClockAlive = false) const;
 
   // Get battery percentage (range 0-100)
   uint16_t getBatteryPercentage() const;