Act as a Creative Frontend Developer. Create a self-contained "Retro Pixel Art Editor" within a single `index.html` file using HTML5, Custom CSS (no Tailwind for this one), and Vanilla JavaScript.

Core Requirements:
1. Canvas/Grid System:
   - Provide a way for the user to select the canvas resolution (e.g., 16x16 or 32x32).
   - Generate a pure DOM-based grid (using flexbox or CSS grid) where each cell acts as a pixel.
2. Drawing Tools:
   - A native color picker `<input type="color">`.
   - A "Draw" mode and an "Eraser" mode.
   - Continuous drawing: The user should be able to click, hold, and drag their mouse across the grid to color multiple cells smoothly (listen to mousedown, mouseenter, and mouseup events).
3. Utility Features:
   - A "Clear Canvas" button.
   - A "Download as PNG" button: Use HTML5 Canvas API in the background to draw the current state of the DOM grid and trigger a download of the resulting image.
4. Styling:
   - Give the app a nostalgic, retro aesthetic (pixelated fonts, boxy UI). 

Output the complete, fully functioning code in one single block. Ensure the mouse-drag drawing logic feels responsive and doesn't miss cells when the mouse moves quickly.