Ship Android app by wrapping CRA build with Capacitor (#20)

Add Capacitor 7 scaffolding (config, android/, npm scripts, .env.mobile),
native back-button/safe-area chrome, icons/splash, and signing docs so
llm-fe ships one codebase to web and Android.

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
 );

llm-fe/src/llm-fe/platform/initNativeShell.js

@@ -0,0 +1,14 @@
+/**
+ * Boot native Capacitor chrome (back button, status bar, keyboard).
+ * Safe to call from index.tsx; no-ops when not running in a Capacitor shell.
+ */
+import { Preferences } from '@capacitor/preferences';
+import { setupAndroidBackButton, setupNativeChrome } from './nativeChrome';
+
+// Side-effect import registers Preferences on window.Capacitor.Plugins for tokenStorage.
+void Preferences;
+
+export async function initNativeShell() {
+  await setupNativeChrome();
+  return setupAndroidBackButton();
+}

llm-fe/src/llm-fe/platform/nativeChrome.js

@@ -0,0 +1,58 @@
+/**
+ * Android hardware back button + keyboard / status-bar insets for Capacitor (#20).
+ * No-ops on web.
+ */
+
+import { isNativePlatform } from './nativePlatform';
+
+/**
+ * Register Android back-button: pop history when possible, else exit app at root.
+ * Call once after React mounts.
+ * @returns {Promise<(() => void) | undefined>} unsubscribe
+ */
+export async function setupAndroidBackButton() {
+  if (!isNativePlatform()) {
+    return undefined;
+  }
+
+  const { App } = await import('@capacitor/app');
+
+  const handle = await App.addListener('backButton', ({ canGoBack }) => {
+    if (canGoBack || (typeof window !== 'undefined' && window.history.length > 1)) {
+      window.history.back();
+      return;
+    }
+    App.exitApp();
+  });
+
+  return () => {
+    handle.remove();
+  };
+}
+
+/**
+ * Configure status bar + keyboard resize so MUI layout respects safe areas.
+ * @returns {Promise<void>}
+ */
+export async function setupNativeChrome() {
+  if (!isNativePlatform()) {
+    return;
+  }
+
+  try {
+    const { StatusBar, Style } = await import('@capacitor/status-bar');
+    await StatusBar.setOverlaysWebView({ overlay: false });
+    await StatusBar.setStyle({ style: Style.Dark });
+  } catch {
+    /* plugin optional at runtime */
+  }
+
+  try {
+    const { Keyboard, KeyboardResize } = await import('@capacitor/keyboard');
+    if (typeof Keyboard.setResizeMode === 'function') {
+      await Keyboard.setResizeMode({ mode: KeyboardResize.Body });
+    }
+  } catch {
+    /* Keyboard.setResizeMode is Android-only; ignore on iOS / missing plugin */
+  }
+}

llm-fe/src/llm-fe/platform/nativeChrome.test.js

@@ -0,0 +1,22 @@
+import { setupAndroidBackButton, setupNativeChrome } from './nativeChrome';
+
+describe('nativeChrome', () => {
+  const originalCapacitor = window.Capacitor;
+
+  afterEach(() => {
+    window.Capacitor = originalCapacitor;
+    jest.resetModules();
+    jest.clearAllMocks();
+  });
+
+  test('setupAndroidBackButton is no-op without Capacitor', async () => {
+    delete window.Capacitor;
+    const unsub = await setupAndroidBackButton();
+    expect(unsub).toBeUndefined();
+  });
+
+  test('setupNativeChrome is no-op without Capacitor', async () => {
+    delete window.Capacitor;
+    await expect(setupNativeChrome()).resolves.toBeUndefined();
+  });
+});

llm-fe/src/llm-fe/platform/nativeSafeArea.css

@@ -0,0 +1,14 @@
+/* Capacitor / notch safe-area insets (#20). Harmless on desktop web. */
+html {
+  padding: env(safe-area-inset-top, 0) env(safe-area-inset-right, 0)
+    env(safe-area-inset-bottom, 0) env(safe-area-inset-left, 0);
+}
+
+body {
+  /* Keyboard plugin resizes body; keep root scrollable within inset. */
+  min-height: 100%;
+}
+
+#root {
+  min-height: 100%;
+}