fix: Use HalPowerManager for battery percentage (#1005)

## Summary

The introduction of `HalGPIO` moved the `BatteryMonitor battery` object
into the member function `HalGPIO::getBatteryPercentage()`.

Then, with the introduction of `HalPowerManager`, this function was
moved to `HalPowerManager::getBatteryPercentage()`.

However, the original `BatteryMonitor battery` object is still utilized
by themes for displaying the battery percentage.

This PR replaces these deprecated uses of `BatteryMonitor battery` with
the new `HalPowerManager::getBatteryPercentage()` function.

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lib/hal/HalPowerManager.h

@@ -34,7 +34,7 @@ class HalPowerManager {
   void startDeepSleep(HalGPIO& gpio) const;
 
   // Get battery percentage (range 0-100)
-  int getBatteryPercentage() const;
+  uint16_t getBatteryPercentage() const;
 
   // RAII helper class to manage power saving locks
   // Usage: create an instance of Lock in a scope to disable power saving, for example when running a task that needs