feat: Phase 28.1.2 — dual-network binding + OkHttp clients
- NetworkBinder: discovers Pi hotspot WiFi (no-internet) and cellular networks
via ConnectivityManager, creates per-network socket-bound OkHttpClients
- DualNetworkTestScreen: real OkHttp calls to 10.42.0.1/api/gps/status and
yaris.ceaf.dev/health, simultaneous "Test Both" gate button
- MainActivity: discovers networks at startup, passes bound clients to screen
- Retrofit interfaces (PiApi, ServerApi) + DTOs (GpsStatus, HealthResponse)
- Uncommented OkHttp/Retrofit/kotlinx-serialization deps in build.gradle.kts