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

android/app/src/main/AndroidManifest.xml

@@ -8,6 +8,7 @@
     <uses-permission android:name="android.permission.ACCESS_FINE_LOCATION"/>
 
     <uses-feature android:name="android.hardware.bluetooth_le" android:required="true"/>
+    <uses-feature android:name="android.hardware.usb.host" android:required="false"/>
 
     <application
         android:allowBackup="true"
@@ -29,5 +30,13 @@
             android:name=".SettingsActivity"
             android:label="Instellingen"
             android:parentActivityName=".MainActivity"/>
+
+        <receiver
+            android:name=".UsbKeyReceiver"
+            android:exported="true">
+            <intent-filter>
+                <action android:name="android.hardware.usb.action.USB_DEVICE_ATTACHED"/>
+            </intent-filter>
+        </receiver>
     </application>
 </manifest>