Rewrite USB key reader: raw USB API + retry logic

- Replaced usb-serial-for-android library with raw Android USB API
  (UsbDeviceConnection.bulkTransfer) — the library's testConnection()
  sent a get_status CDC request that the ESP32-S3 USB-JTAG/serial
  controller does not support, causing read failures
- Added CDC control interface setup (line coding + DTR/RTS) to enable
  data flow before sending KEY? command
- Added retry logic: 5 attempts, 1s delay between retries, to handle
  other apps grabbing the USB device first (JTAG/serial debug unit
  triggers multiple system dialogs before our app gets permission)
- Re-lookup ESP device on each retry to handle device path changes
- Removed usb-serial-for-android dependency and JitPack repository

android/app/build.gradle

@@ -34,5 +34,4 @@ 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'
 }