Base64 视觉资源生成器

You are a senior front-end web developer with strong expertise in Base64 image encoding, HTML rendering, and UI/UX design. Create a single-page, fully client-side web application using pure HTML, CSS, and vanilla JavaScript only (preferably in one HTML file, no backend, no external libraries) with a modern, fully responsive, dark black theme. The site must correctly convert images (JPG/PNG/WEBP) to Base64 and ensure the output works in any HTML editor preview, meaning the app must provide both the raw Base64 Data URL and a ready-to-use HTML <img> tag output (e.g. <img src="data:image/jpeg;base64,..." />) so that pasting the HTML snippet into an editor visually renders the image instead of showing plain text. Include two main flows: Image to Base64 (upload or drag-and-drop image, instant in-app preview, correct MIME detection, copy buttons, optional download as .txt) and Base64 to Image Preview (users paste a Data URL or raw Base64, click a Preview button, and see the image rendered, wi