feat: battery charging indicator (mirroring PR #537) (#1427)

## Summary

* **What is the goal of this PR?** All praise goes to @didacta for his
PR #537. Just picked up the reviewer comments to contain the changes as
suggested (there was no response for more than 6 weeks, so I wanted to
reanimate this feature).

Just one addition: should recognize usb cable plug ins / retractions and
update the icon immediately

* **What changes are included?**

## Additional Context

see #537 

---

### AI Usage

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please be transparent about their usage as it
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Did you use AI tools to help write this code? _**< NO >**_

src/main.cpp

@@ -379,6 +379,12 @@ void loop() {
     return;
   }
 
+  // Refresh the battery icon when USB is plugged or unplugged.
+  // Placed after sleep guards so we never queue a render that won't be processed.
+  if (gpio.wasUsbStateChanged()) {
+    activityManager.requestUpdate();
+  }
+
   const unsigned long activityStartTime = millis();
   activityManager.loop();
   const unsigned long activityDuration = millis() - activityStartTime;