एक्सपेंस ट्रैकर ऐप टेम्पलेट

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

The app: Expense Tracker — an income and expense log with totals, categories and CSV export. Log income and expenses, see totals and export CSV.

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.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.toast(message: string): void — short native toast

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]