Milestone v1.0.0 — Android APK

Release APK built locally (Expo prebuild + Gradle), smoke-tested end to end on an Android 16 emulator.

Prototype status: auth and therapist data are mocked — enter via the labelled Explore the demo button. The AI chat needs an EXPO_PUBLIC_OPENAI_API_KEY at build time; without one it says plainly that it is not live. See the README and docs/milestone-demo.mp4 for the walkthrough.