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 >**_

lib/hal/HalGPIO.cpp

@@ -7,7 +7,14 @@ void HalGPIO::begin() {
   pinMode(UART0_RXD, INPUT);
 }
 
-void HalGPIO::update() { inputMgr.update(); }
+void HalGPIO::update() {
+  inputMgr.update();
+  const bool connected = isUsbConnected();
+  usbStateChanged = (connected != lastUsbConnected);
+  lastUsbConnected = connected;
+}
+
+bool HalGPIO::wasUsbStateChanged() const { return usbStateChanged; }
 
 bool HalGPIO::isPressed(uint8_t buttonIndex) const { return inputMgr.isPressed(buttonIndex); }
 

lib/hal/HalGPIO.h

@@ -23,6 +23,9 @@ class HalGPIO {
   InputManager inputMgr;
 #endif
 
+  bool lastUsbConnected = false;
+  bool usbStateChanged = false;
+
  public:
   HalGPIO() = default;
 
@@ -41,6 +44,9 @@ class HalGPIO {
   // Check if USB is connected
   bool isUsbConnected() const;
 
+  // Returns true once per edge (plug or unplug) since the last update()
+  bool wasUsbStateChanged() const;
+
   enum class WakeupReason { PowerButton, AfterFlash, AfterUSBPower, Other };
 
   WakeupReason getWakeupReason() const;
@@ -54,3 +60,5 @@ class HalGPIO {
   static constexpr uint8_t BTN_DOWN = 5;
   static constexpr uint8_t BTN_POWER = 6;
 };
+
+extern HalGPIO gpio;  // Singleton