Initial commit: Android Tether API app

Exposes phone connection status via HTTP API for tethered devices.

Features:
- Auto-start/stop server when tethering is enabled/disabled
- REST API with /status and /health endpoints
- Connection type, signal strength, carrier, battery info
- Foreground service with persistent notification

Stack: Kotlin 2.3, AGP 8.13, Gradle 8.13, compileSdk 36

app/proguard-rules.pro

@@ -0,0 +1,8 @@
+# NanoHTTPD
+-keep class fi.iki.elonen.** { *; }
+-keep class org.nanohttpd.** { *; }
+
+# Gson
+-keepattributes Signature
+-keepattributes *Annotation*
+-keep class dev.itsh.tetherapi.data.** { *; }