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/src/index.tsx

@@ -1,8 +1,10 @@
 import React from 'react';
 import ReactDOM from 'react-dom/client';
 import './index.css';
+import './llm-fe/platform/nativeSafeArea.css';
 import App from './App';
 import AppRouter from './llm-fe/platform/AppRouter';
+import { initNativeShell } from './llm-fe/platform/initNativeShell';
 import { AuthProvider } from './llm-fe/contexts/AuthContext';
 import { AccountProvider } from './llm-fe/contexts/AccountContext';
 import { WebSocketProvider } from './llm-fe/contexts/WebSocketContext';
@@ -11,6 +13,8 @@ import { ConversationProvider } from './llm-fe/contexts/ConversationContext';
 import { MessageProvider } from './llm-fe/contexts/MessageContext';
 import { PreferenceProvider } from './llm-fe/contexts/PreferencesContext';
 
+void initNativeShell();
+
 const root = ReactDOM.createRoot(
   document.getElementById('root') as HTMLElement
 );