Use a saturated violet (#5938e5) for the primary button background in
both themes so white text reads cleanly — previously dark mode used
the lighter --accent (#a48bff), giving low contrast against white.
Also raise the gradient h1 line-height from 1.04 to 1.18 and add
padding-bottom: .08em so the gradient text-clip stops chopping the
descenders on letters like g/y/p.
Port gogcli's custom static-site builder (scripts/build-docs-site.mjs +
scripts/docs-site-assets.mjs) and adapt branding to songsee — spectral
gradient hero, viz-mode pill row, JetBrains Mono code blocks, sidebar
nav with search, dark/light theme toggle, per-page TOC, and link
validation at build time.
Wire it up via 'make docs-site' and a pages.yml workflow that uploads
dist/docs-site as the Pages artifact. GitHub Pages source is now
build_type=workflow (was legacy/Jekyll).
