Refine USB transport flow

- replace Android USB dependency with app-owned USB host implementation\n- restore BLE-first scanner flow with USB secondary action\n- tighten Web Serial key handling and disconnect logging\n\nTODO (follow-up):\n- review non-English localization copy for tone and consistency\n- trim remaining unused/awkward localization strings introduced during USB UI changes

android/app/build.gradle.kts

@@ -16,7 +16,7 @@ if (keystorePropertiesFile.exists()) {
 android {
     namespace = "com.meshcore.meshcore_open"
     compileSdk = flutter.compileSdkVersion
-    ndkVersion = "29.0.14206865"
+    ndkVersion = flutter.ndkVersion
 
     compileOptions {
         sourceCompatibility = JavaVersion.VERSION_17
@@ -84,5 +84,4 @@ flutter {
 
 dependencies {
     coreLibraryDesugaring("com.android.tools:desugar_jdk_libs:2.1.4")
-    implementation("com.github.mik3y:usb-serial-for-android:3.9.0")
 }