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/.gitignore

@@ -11,12 +11,25 @@
 # production
 /build
 
+# Capacitor / Android Gradle artifacts (keep android/ source; ignore local builds)
+/android/.gradle/
+/android/app/build/
+/android/build/
+/android/captures/
+/android/local.properties
+/android/*.iml
+/android/app/*.iml
+/android/.idea/
+*.keystore
+*.jks
+
 # misc
 .DS_Store
 .env.local
 .env.development.local
 .env.test.local
 .env.production.local
+.env.mobile.local
 
 npm-debug.log*
 yarn-debug.log*