android(audio): Speaker button toggles earpiece↔speaker via JNI (WIP, untested)

Build 9e37201 confirmed on-device that Usage::VoiceCommunication +
MODE_IN_COMMUNICATION + speakerphoneOn=false routes Oboe playout to the
handset earpiece and the callback drains the ring correctly. Next step:
let the user flip speakerphoneOn at runtime so the existing Speaker
button actually switches audio routing instead of just gating writes.

- Cargo.toml (android target): pull in `jni = 0.21` and
  `ndk-context = 0.1`. Both are already transitively in the lockfile
  via Tauri/Wry, so this just promotes them to direct deps.
- desktop/src-tauri/src/android_audio.rs: new module. Grabs the JavaVM +
  current Activity from `ndk_context::android_context()`, attaches a
  JNI thread, calls `activity.getSystemService("audio")` to get the
  AudioManager, and exposes `set_speakerphone(bool)` +
  `is_speakerphone_on()` helpers that call the AudioManager method of
  the same name. All gated behind `#[cfg(target_os = "android")]`.
- lib.rs: adds `mod android_audio;` (android only), two new Tauri
  commands `set_speakerphone(on)` and `is_speakerphone_on()` — desktop
  gets no-op stubs so the same frontend invoke() works everywhere.
  Both registered in the invoke_handler.
- desktop/src/main.ts: the Speaker button (previously toggled the
  playout-write gate via `toggle_speaker`) now calls `set_speakerphone`
  and reads back the new routing state. Labels switched from
  "Spk" / "Spk Off" to "Earpiece" / "Speaker" so users can't be
  confused into thinking clicking turns audio off. pollStatus no longer
  clobbers the spkBtn label based on engine spk_muted, since the two
  concepts are now decoupled.

WIP because this has NOT been built or tested yet — committing at night
to save the work. Tomorrow: build #50 with this change, smoke-test the
Handset↔Speaker toggle, then move on to call history + last-contacts UI
and the Speaker-button mute bug on the other phone.

desktop/src-tauri/src/android_audio.rs

@@ -0,0 +1,98 @@
+//! Runtime bridge to Android's `AudioManager` for in-call audio routing.
+//!
+//! We own a quinn+Oboe VoIP pipeline entirely from Rust, but routing the
+//! playout stream between earpiece / loudspeaker / Bluetooth headset has to
+//! happen at the JVM level because those toggles are AudioManager-only.
+//! This module uses the global JavaVM handle that `ndk_context` exposes
+//! (populated by Tauri's mobile runtime) + the `jni` crate to reach into
+//! the Android framework without needing a Tauri plugin.
+//!
+//! All callers must be inside an Android target (`#[cfg(target_os = "android")]`).
+
+#![cfg(target_os = "android")]
+
+use jni::objects::{JObject, JString, JValue};
+use jni::JavaVM;
+
+/// Grab the JavaVM + current Activity from the ndk_context that Tauri's
+/// mobile runtime sets up at process startup.
+fn jvm_and_activity() -> Result<(JavaVM, JObject<'static>), String> {
+    let ctx = ndk_context::android_context();
+    let vm_ptr = ctx.vm().cast();
+    if vm_ptr.is_null() {
+        return Err("ndk_context: JavaVM pointer is null".into());
+    }
+    let vm = unsafe { JavaVM::from_raw(vm_ptr) }
+        .map_err(|e| format!("JavaVM::from_raw: {e}"))?;
+    let activity_ptr = ctx.context().cast();
+    if activity_ptr.is_null() {
+        return Err("ndk_context: activity pointer is null".into());
+    }
+    // SAFETY: ndk_context guarantees the pointer lives for the process
+    // lifetime; we wrap it as a JObject<'static> for convenience.
+    let activity: JObject<'static> = unsafe { JObject::from_raw(activity_ptr) };
+    Ok((vm, activity))
+}
+
+/// Get Android's `AudioManager` via `activity.getSystemService("audio")`.
+fn audio_manager<'local>(
+    env: &mut jni::AttachGuard<'local>,
+    activity: &JObject<'local>,
+) -> Result<JObject<'local>, String> {
+    let svc_name: JString<'local> = env
+        .new_string("audio")
+        .map_err(|e| format!("new_string(audio): {e}"))?;
+    let am = env
+        .call_method(
+            activity,
+            "getSystemService",
+            "(Ljava/lang/String;)Ljava/lang/Object;",
+            &[JValue::Object(&svc_name)],
+        )
+        .and_then(|v| v.l())
+        .map_err(|e| format!("getSystemService(audio): {e}"))?;
+    if am.is_null() {
+        return Err("getSystemService returned null".into());
+    }
+    Ok(am)
+}
+
+/// Switch between loud speaker (`true`) and earpiece/handset (`false`).
+///
+/// Calls `AudioManager.setSpeakerphoneOn(on)` on the JVM. Requires that
+/// the audio mode is already `MODE_IN_COMMUNICATION` — MainActivity.kt
+/// sets this at startup, so by the time a call is up this is always true.
+pub fn set_speakerphone(on: bool) -> Result<(), String> {
+    let (vm, activity) = jvm_and_activity()?;
+    let mut env = vm
+        .attach_current_thread()
+        .map_err(|e| format!("attach_current_thread: {e}"))?;
+    let am = audio_manager(&mut env, &activity)?;
+
+    env.call_method(
+        &am,
+        "setSpeakerphoneOn",
+        "(Z)V",
+        &[JValue::Bool(if on { 1 } else { 0 })],
+    )
+    .map_err(|e| format!("setSpeakerphoneOn({on}): {e}"))?;
+
+    tracing::info!(on, "AudioManager.setSpeakerphoneOn");
+    Ok(())
+}
+
+/// Query the current speakerphone state. Returns true if routing is on the
+/// loud speaker, false if on earpiece / BT headset / wired headset.
+pub fn is_speakerphone_on() -> Result<bool, String> {
+    let (vm, activity) = jvm_and_activity()?;
+    let mut env = vm
+        .attach_current_thread()
+        .map_err(|e| format!("attach_current_thread: {e}"))?;
+    let am = audio_manager(&mut env, &activity)?;
+
+    let on = env
+        .call_method(&am, "isSpeakerphoneOn", "()Z", &[])
+        .and_then(|v| v.z())
+        .map_err(|e| format!("isSpeakerphoneOn: {e}"))?;
+    Ok(on)
+}