Create a modern, responsive YouTube Thumbnail Downloader web app. The site must let users paste a YouTube video URL and instantly preview and download available thumbnails (including 4K when available). Deliver separate files: index.html, styles.css, app.js. Provide production-ready, accessible, SEO-friendly code and clear inline comments. Design & visual style: Clean, professional UI with a subtle gradient primary palette (eg. #fff95b to #ff930f) and smooth neumorphic / glassy cards. Support 4K preview (high-res image display, with download button that saves original resolution if available). Use modern typography (system stack + Google Font fallback). Add tasteful micro-animations: header fade, input focus glow, button hover, thumbnail hover scale, download progress animation. Use CSS transitions and minimal JS for animations. Provide both desktop and mobile layouts (responsive grid); ensure it looks great on 4K monitors and mobiles. Header & Footer: Header: logo (SVG), site name, navigation links: Home, About, How it works, Privacy Policy, Contact. Sticky on scroll with subtle backdrop blur. Footer: quick links, short copyright, contact email, social icons (SVG), small note about respecting YouTube TOS. Core features & UX: Input area for YouTube URL with placeholder https://www.youtube.com/watch?v=XXXXXXXXXXX. Validate input and show friendly error messages. On submit, extract video ID client-side and fetch thumbnail URLs (standard YouTube patterns + high-res heuristics). No server required for basic functionality — just create download links for known thumbnail endpoints (document logic in comments). Preview area showing multiple sizes (default thumbnails, hqdefault, sddefault, maxresdefault for 4K). For each thumbnail: show resolution label, filesize estimate (if obtainable), a Preview (lazy-loaded), and a Download button. If maxresdefault not available, detect 404 and disable 4K with a tooltip explaining "4K not available for this video". Provide a Download All button that opens a zipped download (optional) or triggers individual downloads sequentially. If zipping client-side, use a small library (mention as optional dependency). Accessibility: keyboard focus states, aria labels, visible form labels, contrast, and alt text for images. Interaction & animations: Input gains a glowing border on focus. Loading state: animated progress bar or spinner while checking images. Thumbnail cards animate in (slide + fade). Download button shows a short success microcopy (toast) and a check animation once download starts. Instructions / “How to download” section: Add a short step-by-step block (3 steps): 1) Paste YouTube link, 2) Choose resolution, 3) Click Download. Provide small screenshots or icons. Include a FAQ blurb about copyright and using thumbnails properly. Code & structure requirements: Provide three separate files only: index.html, styles.css, app.js. No build step required; vanilla HTML/CSS/JS. Keep code modular and well commented. index.html must contain meta tags (Open Graph, meta description), structured data snippet (JSON-LD) for a WebApplication, and semantic markup. styles.css must use CSS variables for theme, responsive breakpoints, and include all animations. Keep styles safe for 4K displays (use max-width containers). app.js must: validate and parse YouTube URLs robustly (support youtu.be, youtube.com/watch?v=, youtube.com/shorts/), build thumbnail URLs, test availability (via fetch head request or image onerror), update UI, and manage downloads. Add comments explaining the logic and fallback behaviour. Performance & privacy: Lazy-load thumbnails. Minimise external requests. Do not send user video URLs to any external server. Make everything client-side. If any external library is used (zip or tiny helper), list it and provide CDN fallback. Extras (nice to have): Drag & drop support for URLs. Copy direct image link button. Theme toggle (light/dark). Small analytics placeholder (commented out) and a privacy note. Deliverables: index.html — full semantic HTML with header, main (input + preview + instructions), footer, and linked CSS/JS. styles.css — complete style file with animations and responsive rules. app.js — completed JS with comments and no external build. A README block at the top of index.html or a separate readme comment describing file usage, browser support, and notes on 4K availability limitations. Tone of UI text: friendly, concise British English (eg. “Paste a YouTube link to fetch thumbnails”). Security & legal note (display somewhere small): remind users to respect YouTube’s Terms of Service and copyright when reusing thumbnails.