feat: Allow screenshot retrieval from device (#820)

## Summary

* Add a small loop in main to be able to receive external commands,
currently being sent via the debugging_monitor
* Implemented command: cmd:SCREENSHOT sends the currently displayed
screen to the monitor, which will then store it to screenshot.bmp

## Additional Context

I was getting annoyed with taking tilted/unsharp photos of the device
screen, so I added the ability to press Enter during the monitor
execution and type SCREENSHOT to send a command. Could be extended in
the future

[screenshot.bmp](https://github.com/user-attachments/files/25213230/screenshot.bmp)

---

### 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/main.cpp

@@ -369,6 +369,21 @@ void loop() {
     lastMemPrint = millis();
   }
 
+  // Handle incoming serial commands
+  if (Serial.available() > 0) {
+    String line = Serial.readStringUntil('\n');
+    if (line.startsWith("CMD:")) {
+      String cmd = line.substring(4);
+      cmd.trim();
+      if (cmd == "SCREENSHOT") {
+        Serial.printf("SCREENSHOT_START:%d\n", HalDisplay::BUFFER_SIZE);
+        uint8_t* buf = display.getFrameBuffer();
+        Serial.write(buf, HalDisplay::BUFFER_SIZE);
+        Serial.printf("SCREENSHOT_END\n");
+      }
+    }
+  }
+
   // Check for any user activity (button press or release) or active background work
   static unsigned long lastActivityTime = millis();
   if (gpio.wasAnyPressed() || gpio.wasAnyReleased() || (currentActivity && currentActivity->preventAutoSleep())) {