phase 3(android): unify connect/disconnect/toggle_*/get_status commands

Step 3 of the Tauri Android rewrite was still returning "audio backend not
yet wired on Android (step 3)" because the cfg-gated Android stubs for
connect/disconnect/toggle_mic/toggle_speaker/get_status were shadowing the
real commands. Now that CallEngine::start() has a real Android body (phase
3, commit fdbe502), the gates are unnecessary.

- Drop the #[cfg(not(target_os = "android"))] gates from all five
  engine-backed Tauri commands.
- Delete the Android stub block (~50 LOC of "not connected" boilerplate).
- Ungate `use engine::CallEngine;` and the AppState.engine field so both
  targets share the same Mutex<Option<CallEngine>>.
- CallEngine::stop() now calls crate::wzp_native::audio_stop() on Android so
  the mic + speaker are released between calls, matching the desktop
  behaviour where dropping _audio_handle tears down CPAL.

Direct-call flow on Android: peer sends DirectCallOffer → user accepts via
answer_call → relay sends signal "setup" event → main.ts auto-invokes
connect(relay, room) → CallEngine::start() runs the Android branch →
wzp_native::audio_start() brings up Oboe → send/recv tasks stream PCM
through the dlopen boundary.

desktop/src-tauri/src/engine.rs

@@ -719,5 +719,13 @@ impl CallEngine {
     pub async fn stop(self) {
         self.running.store(false, Ordering::SeqCst);
         self.transport.close().await.ok();
+        // On Android, the Oboe capture/playout streams live inside the
+        // wzp-native cdylib as a process-global singleton. Explicitly stop
+        // them here so the mic + speaker are released between calls, matching
+        // the desktop behaviour where dropping _audio_handle tears down CPAL.
+        #[cfg(target_os = "android")]
+        {
+            crate::wzp_native::audio_stop();
+        }
     }
 }