feat: Android Compose client for the unibus bus

Thin Jetpack Compose UI over the real Go client (pkg/client) compiled to a
gomobile .aar, so the phone speaks NATS and runs the same end-to-end crypto as
any other peer. Connect, create room (nats/matrix), publish and live-receive.

.gitignore

@@ -0,0 +1,11 @@
+.gradle/
+build/
+local.properties
+*.iml
+.idea/
+captures/
+.externalNativeBuild/
+.cxx/
+*.apk
+*.aab
+# NOTE: app/src/test/snapshots/ is committed (Roborazzi goldens are test refs).