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/build.gradle

@@ -34,4 +34,5 @@ android {
 dependencies {
     implementation 'androidx.core:core-ktx:1.12.0'
     implementation 'androidx.appcompat:appcompat:1.6.1'
+    implementation 'com.github.mik3y:usb-serial-for-android:3.7.3'
 }