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/main.ts

@@ -510,8 +510,25 @@ function showConnectScreen() {
 micBtn.addEventListener("click", async () => {
   try { const m: boolean = await invoke("toggle_mic"); micBtn.classList.toggle("muted", m); micIcon.textContent = m ? "Mic Off" : "Mic"; } catch {}
 });
+
+// Speaker routing (Android) — toggles AudioManager.setSpeakerphoneOn so the
+// same Oboe VoiceCommunication stream swaps between earpiece and
+// loudspeaker without restarting. Desktop callers get a no-op command so
+// the same UI works everywhere.
+let speakerphoneOn = false;
+function updateSpkLabel() {
+  spkBtn.classList.toggle("muted", !speakerphoneOn);
+  spkIcon.textContent = speakerphoneOn ? "Speaker" : "Earpiece";
+}
 spkBtn.addEventListener("click", async () => {
-  try { const m: boolean = await invoke("toggle_speaker"); spkBtn.classList.toggle("muted", m); spkIcon.textContent = m ? "Spk Off" : "Spk"; } catch {}
+  const next = !speakerphoneOn;
+  try {
+    await invoke("set_speakerphone", { on: next });
+    speakerphoneOn = next;
+    updateSpkLabel();
+  } catch (e) {
+    console.error("set_speakerphone failed:", e);
+  }
 });
 hangupBtn.addEventListener("click", async () => {
   userDisconnected = true;
@@ -571,8 +588,9 @@ async function pollStatus() {
 
     micBtn.classList.toggle("muted", st.mic_muted);
     micIcon.textContent = st.mic_muted ? "Mic Off" : "Mic";
-    spkBtn.classList.toggle("muted", st.spk_muted);
-    spkIcon.textContent = st.spk_muted ? "Spk Off" : "Spk";
+    // NB: spkBtn label is driven by the Android audio routing state
+    // (speakerphoneOn / updateSpkLabel), not by the engine's spk_muted.
+    // Skip that here so pollStatus doesn't clobber the routing UI.
     callTimer.textContent = formatDuration(st.call_duration_secs);
 
     const rms = st.audio_level;