Android: remote transcription via whisper.cpp server (Phase 2)

- new Transcriber interface makes the recorder service engine-agnostic;
  WhisperEngine (local JNI) and new RemoteWhisperEngine (POST /inference,
  verbose_json, in-memory WAV upload via new WavEncoder) implement it
- SessionConfig carries serverUrl; when set, BOTH the live pass and the
  final pass transcribe remotely (server owns the model, e.g. large-v3
  on GPU); local engine remains the offline fallback
- UI: Transcribe on: phone/server dropdown + server URL field,
  persisted in SharedPreferences; model controls grey out in server
  mode; manual Transcribe also routes to the server
- network security config permits cleartext HTTP for user-configured
  LAN/tailnet servers (documented; HTTPS works either way)
- WavEncoder round-trip unit test (11 total green); validated phone ->
  Tailscale -> R9700 with large-v3 before the app-side change

android/core/whisper/build.gradle.kts

@@ -30,6 +30,7 @@ android {
 }
 
 dependencies {
+    implementation(project(":core:recording"))   // WavEncoder/Audio for uploads
     // TranscriptFiles is pure Kotlin and unit tested on the JVM.
     testImplementation(libs.junit)
 }