Builds priv/static/squatch_mail.css by concatenating
assets/css/fonts.css (the base64-embedded Bebas Neue / Space Mono
@font-face rules) with the hand-written assets/css/squatch_mail.css.
The dashboard's CSS is plain, hand-authored CSS (see the moduledoc note in
assets/css/squatch_mail.css for why this project skips Tailwind/esbuild's
CSS bundling) — there is no build step for it beyond this concatenation,
which exists so the giant font data-URI blobs live in their own generated
file instead of cluttering the stylesheet humans edit, while the shipped
bundle stays a single self-contained request.