fix: signal mode UI + place_call via stored signal transport

- Don't set callState for signal-only states (prevents auto-join room)
- Store signal transport + fingerprint in EngineState after registration
- place_call/answer_call send directly via signal transport (not command channel)
- Spawn small threads for async signal sends (non-blocking)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

android/app/src/main/java/com/wzp/ui/call/CallViewModel.kt

@@ -690,12 +690,12 @@ class CallViewModel : ViewModel(), WzpCallback {
                         val s = CallStats.fromJson(json)
                         lastCallDuration = s.durationSecs
                         _stats.value = s
-                        if (s.state != 0) {
-                            _callState.value = s.state
-                        }
                         // Track signal state changes for direct calling
                         if (s.state in 5..7) {
                             _signalState.value = s.state
+                            // Don't update callState for signal-only states
+                        } else if (s.state != 0) {
+                            _callState.value = s.state
                         }
                         // Incoming call detection
                         if (s.state == 7) { // IncomingCall