Add OG image generation script and PNG

Add a render-og script (scripts/render-og.mjs) that uses sharp to rasterize public/og-card.svg to public/og-card.png and an npm script "build:og" to run it. Commit the generated og-card.png and update public/index.html to reference the PNG (with image type and dimensions) for social previews. Also update the SVG artwork (styling/text) and add sharp as a devDependency (package.json + package-lock.json updates). This ensures a consistent raster OG image for platforms that require PNG and supplies a reproducible build step.

Author: willwashburn