fix: Android signal thread stack overflow + add version marker to UI

- Spawn signaling on dedicated thread with 4MB stack instead of using
  Android's IO dispatcher thread (insufficient stack for tokio + QUIC)
- Add "direct-call-v1" version marker to home screen subtitle

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

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

@@ -165,7 +165,7 @@ fun InCallScreen(
                     color = Color.White
                 )
                 Text(
-                    text = "ENCRYPTED VOICE",
+                    text = "ENCRYPTED VOICE  \u2022  direct-call-v1",
                     style = MaterialTheme.typography.labelSmall.copy(letterSpacing = 3.sp),
                     color = TextDim
                 )