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

While CrossPoint doesn't have restrictions on AI tools in contributing,
please be transparent about their usage as it
helps set the right context for reviewers.

Did you use AI tools to help write this code? _**< NO >**_

src/components/themes/BaseTheme.cpp

@@ -5,6 +5,7 @@
 #include <HalStorage.h>
 #include <Logging.h>
 
+#include <algorithm>
 #include <cstdint>
 #include <string>
 
@@ -34,13 +35,40 @@ void drawBatteryIcon(const GfxRenderer& renderer, int x, int y, int battWidth, i
   renderer.drawPixel(x + battWidth - 1, y + rectHeight - 4);
   renderer.drawLine(x + battWidth - 0, y + 4, x + battWidth - 0, y + rectHeight - 5);
 
+  const bool charging = gpio.isUsbConnected();
+
   // The +1 is to round up, so that we always fill at least one pixel
-  int filledWidth = percentage * (battWidth - 5) / 100 + 1;
-  if (filledWidth > battWidth - 5) {
-    filledWidth = battWidth - 5;  // Ensure we don't overflow
+  const int maxFillWidth = battWidth - 5;
+  const int fillHeight = rectHeight - 4;
+  if (maxFillWidth <= 0 || fillHeight <= 0) {
+    return;
+  }
+  int filledWidth = percentage * maxFillWidth / 100 + 1;
+  if (filledWidth > maxFillWidth) {
+    filledWidth = maxFillWidth;
   }
 
-  renderer.fillRect(x + 2, y + 2, filledWidth, rectHeight - 4);
+  // When charging, ensure minimum fill so lightning bolt is fully visible
+  constexpr int minFillForBolt = 8;
+  if (charging && filledWidth < minFillForBolt) {
+    filledWidth = std::min(minFillForBolt, maxFillWidth);
+  }
+
+  renderer.fillRect(x + 2, y + 2, filledWidth, fillHeight);
+
+  // Draw lightning bolt when charging (white/inverted on black fill for visibility)
+  if (charging) {
+    const int boltX = x + 4;
+    const int boltY = y + 2;
+    renderer.drawLine(boltX + 4, boltY + 0, boltX + 5, boltY + 0, false);
+    renderer.drawLine(boltX + 3, boltY + 1, boltX + 4, boltY + 1, false);
+    renderer.drawLine(boltX + 2, boltY + 2, boltX + 5, boltY + 2, false);
+    renderer.drawLine(boltX + 3, boltY + 3, boltX + 4, boltY + 3, false);
+    renderer.drawLine(boltX + 2, boltY + 4, boltX + 3, boltY + 4, false);
+    renderer.drawLine(boltX + 1, boltY + 5, boltX + 4, boltY + 5, false);
+    renderer.drawLine(boltX + 2, boltY + 6, boltX + 3, boltY + 6, false);
+    renderer.drawLine(boltX + 1, boltY + 7, boltX + 2, boltY + 7, false);
+  }
 }
 }  // namespace