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

The app: Blank App — a clean, minimal starting point with a header, a card and a primary button. Clean mobile boilerplate with safe areas, theme color and a FreeApp ready hook.

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.statusBar(hexColor: string): void — e.g. '#2563EB'
- FreeApp.device(): {manufacturer, model, brand, androidVersion, sdkInt, language, screenWidth, screenHeight, density, isTablet, darkMode}

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]