Add USB auto-read key transfer + UI redesign

USB auto-read:
- ESP responds to 'KEY?\n' over USB serial with 'HMAC_KEY:<hex>'
- Android UsbKeyReceiver (manifest BroadcastReceiver) catches USB_DEVICE_ATTACHED
- Filters on Espressif vendor 0x303a, requests USB permission
- Opens CDC-ACM serial (115200), sends KEY?, parses response, saves to SharedPreferences
- Works even when app not in foreground
- usb-serial-for-android v3.7.3 via JitPack

UI redesign:
- activity_main.xml: FrameLayout with settings icon top-right, relay button center, status bottom-left (14sp)
- Scan & Connect button moved to SettingsActivity
- SettingsActivity: hint text 'Voer de HMAC sleutel in of sluit de relaismodule aan op de telefoon'
- SettingsActivity: scan button saves key + returns RESULT_OK to trigger scanning
- MainActivity: removed scanButton, auto-scan on onResume when key is set
- Back button removed from settings (ActionBar up navigation only)

README updated with USB protocol docs, new UI description, and updated setup guide

src/main.cpp

@@ -158,6 +158,23 @@ void setup() {
 }
 
 void loop() {
+    if (Serial.available()) {
+        static String serialBuf;
+        char c = Serial.read();
+        serialBuf += c;
+        if (c == '\n') {
+            serialBuf.trim();
+            if (serialBuf == "KEY?") {
+                Serial.print("HMAC_KEY:");
+                for (int i = 0; i < 32; i++) {
+                    Serial.printf("%02X", HMAC_KEY[i]);
+                }
+                Serial.println();
+            }
+            serialBuf = "";
+        }
+    }
+
     int sensorState = digitalRead(SENSOR_PIN);
 
     bool touchActive = (sensorState == LOW);