Move CSS, JS, favicon files to static/ directory to separate
compiled-in assets from Askama templates. Update all include paths,
build.rs, flake.nix, and Tailwind @source directives.
Replace the checked-in styles.css with a build-time Tailwind CSS
pipeline. input.css defines CSS custom properties for light/dark
themes, mapped to Tailwind utilities via @theme. build.rs runs
the Tailwind CLI to generate styles.css at compile time.