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>

Cargo.lock

@@ -4370,6 +4370,7 @@ dependencies = [
  "async-trait",
  "axum 0.7.9",
  "bytes",
+ "chrono",
  "dirs",
  "futures-util",
  "prometheus",