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Build a single-file mobile web app (index.html with inline CSS/JS) for Android WebView.

The app: Native Features — a showcase grid of buttons that each demonstrate one native feature. Every FreeApp API: vibrate, torch, notify, share, battery, TTS and more.

Requirements:
- Mobile-first, touch-friendly UI (min 44px tap targets), works in portrait, supports light and dark mode.
- Use <meta name="viewport" content="width=device-width, initial-scale=1, viewport-fit=cover"> and pad for env(safe-area-inset-*).
- Must work fully offline: no CDN or external scripts, fonts or images; persist data in localStorage.

The page runs inside an Android app that injects a native bridge as window.FreeApp. Available APIs:
- FreeApp.toast(message: string): void — short native toast
- FreeApp.vibrate(ms?: number | number[]): void — one-shot buzz (default 50 ms) or an [off, on, off, on…] pattern
- FreeApp.share(text: string, title?: string): void — Android share sheet
- FreeApp.copy(text: string): void — copy to clipboard
- FreeApp.device(): {manufacturer, model, brand, androidVersion, sdkInt, language, screenWidth, screenHeight, density, isTablet, darkMode}
- FreeApp.app(): {packageName, versionName, versionCode, name, firstInstall}
- FreeApp.battery(): {level: number /* 0-100 */, charging: boolean}
- FreeApp.online(): boolean
- FreeApp.network(): 'wifi' | 'cellular' | 'ethernet' | 'vpn' | 'other' | 'none' | 'unknown'
- FreeApp.torch(on: boolean): boolean — flashlight, returns false if no flash
- FreeApp.keepAwake(on: boolean): void — keep the screen on
- FreeApp.brightness(level: number): void — 0.0–1.0 for this window, negative restores system brightness
- FreeApp.statusBar(hexColor: string): void — e.g. '#2563EB'
- FreeApp.fullscreen(on: boolean): void — hide system bars (immersive)
- FreeApp.orientation(value: 'portrait' | 'landscape' | 'sensor_portrait' | 'sensor_landscape' | 'sensor' | 'unspecified'): void
- FreeApp.notify(title: string, body?: string): void — instant local notification (asks permission on Android 13+)
- FreeApp.speak(text: string, lang?: string): void — native text-to-speech, lang like 'en-US'
- FreeApp.print(): void — print / save the page as PDF
- FreeApp.store.set(key: string, value: any): void — persistent native key/value (JSON)
- FreeApp.store.get(key: string): any | null
- FreeApp.saveFile(name: string, mime: string, data: string /* base64 or data: URL */ | Blob): Promise<boolean> — saves to Downloads; FreeApp.on('saved', d => …) gives {name, ok, location}
- FreeApp.requestPermission(name: 'camera' | 'microphone' | 'location' | 'notifications'): Promise<boolean>
- FreeApp.openSettings(): void — open this app's system settings page

Rules for the bridge:
- Always guard calls with `window.FreeApp &&` (or `if (window.FreeApp) { … }`) and provide a browser fallback so the page also works in a normal browser.
- The bridge may be injected after your script runs: do native setup inside `window.addEventListener('freeappready', …)` and also run it immediately if window.FreeApp already exists.
- Do not use alert/confirm for normal feedback; prefer FreeApp.toast when available.

Return the complete index.html in one code block.

My idea: [DESCRIBE YOUR APP IDEA HERE]