feat: landing page support, edit-this-page links, and body font-weight fix

[DanFlannel]

Summary

Two features that close the remaining gaps vs Docusaurus, plus a site-wide font fix they surfaced.

Landing page (content/home.mdx) — presence-gated: drop the file in and / renders a
prerendered landing page (TopNav + footer, no sidebar); delete it and / redirects to /docs
as before. Ships four MDX components (Hero, FeatureGrid, Feature, LinkButton — all real
crawlable anchors), a demo home page for this repo, and full pipeline integration:

• home is a reserved slug — excluded from sidebar/search/manifest/llms.txt, but
  prerendered with its own OG card and a / sitemap entry.
• The prerendered home is written to dist/index.html, which doubles as the SPA fallback for
  non-prerendered URLs. Every prerendered page therefore carries a
  <meta name="capsa-prerendered"> route marker; the client hydrates only on a marker match
  and falls back to a clean client render otherwise (404s and /docs/api covered by e2e).
• The brand wordmark in the top bar now links home from every page (/ with a landing page,
  /docs without) — SPA-fast, cmd-clickable.

Edit this page — every doc links to GITHUB_URL/edit/<branch>/content/<slug>.mdx in two
places: a PageActions menu item and a visible link below the article. New optional
VITE_GITHUB_BRANCH (default main).

Fix: body text rendered bold site-wide — Tamagui resolves font weight from the
size-matched key in the font config (fontSize="$5" → weight['5']), and the map escalated
with size, so every 16px+ paragraph rendered at weight 700. The weight map is now flat 400 at
every size; headings keep their explicit weights. Also adds a body font-family fallback so
stray text nodes never fall to serif.

Type of change

• Bug fix
• New feature
• Docs / content

Checklist

• pnpm typecheck passes
• pnpm lint passes
• pnpm format:check passes (run pnpm format to fix)
• pnpm build succeeds
• pnpm check:links passes
• Docs/examples updated if behavior changed

Notes

• Behavioral note: non-prerendered URLs are now served the landing-page HTML by static
  hosts (previously the empty shell). Head meta is identical (site defaults) and the route
  marker guarantees a clean client render — no user-visible change, but worth knowing when
  debugging view-source.
• Visual change: all body text drops from 700 → 400 weight — this is the intended look;
  don't tune themes against the old bold rendering.
• e2e grew 16 → 21 tests (landing prerender + hydration, SPA-fallback safety, reserved-slug
  exclusion, edit links + brand-home navigation). PRERENDER=0 rollback re-verified.
• New guide: Guides → Landing Page; config docs + README + .env.example updated.

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