Ship Android app by wrapping the web build with Capacitor (#20) (#28)

## Summary
- Closes #20
- Add Capacitor 7 to `llm-fe/` (`capacitor.config.ts`, appId `com.aimloperations.chat`, `webDir: build`) and commit the generated `android/` project
- npm scripts: `build:mobile` (`.env.mobile` + CRA build + `cap sync`), `android:open`, `android:sync`, `assets:generate`
- `.env.mobile` defaults to prod backend; override independently of web deploys
- Native chrome: Android back button (history / exit at root), status bar + keyboard resize, safe-area CSS; Preferences registered for JWT mirror (#22)
- Icons/splash via `@capacitor/assets`; signing via optional `keystore.properties`; docs in `llm-fe/ANDROID.md` + root README

Blockers #22 / #23 / #24 already on master.

## Test plan
- [x] `npm run test:ci` (79 tests)
- [x] `npm run build` + `npx cap add android` / `cap sync`
- [ ] `npm run build:mobile` + open in Android Studio on a machine with SDK
- [ ] Emulator/device QA: login, chat stream, theme, back button, keyboard/safe-area, WS resume
- [ ] Generate upload keystore outside repo; `./gradlew bundleRelease` for internal testing trackReviewed-on: #28

llm-fe/.env.mobile

@@ -0,0 +1,6 @@
+# Mobile (Capacitor) production endpoints — same as .env.production by default.
+# Override here to point Android/iOS shells at beta without changing web deploys:
+#   REACT_APP_BACKEND_REST_API_BASE_URL=https://beta.chatbackend.aimloperations.com/api/
+#   REACT_APP_BACKEND_WS_API_BASE_URL=wss://beta.chatbackend.aimloperations.com/ws/chat_again/
+REACT_APP_BACKEND_REST_API_BASE_URL=https://chatbackend.aimloperations.com/api/
+REACT_APP_BACKEND_WS_API_BASE_URL=wss://chatbackend.aimloperations.com/ws/chat_again/