feat(site): 🛬 PCBJam landing page — waitlist, sections, Tailwind

Replace the placeholder homepage with the PCBJam waitlist conversion page
(13 sections from the copy spec), shared chrome, and a real email-capture
backend, keeping the site zero-runtime-JS / zero-web-font.

- Tailwind v4 via @tailwindcss/postcss (the Vite plugin breaks on Astro 6
  rolldown-vite); Preflight skipped; brand tokens in :root + @theme inline.
- Waitlist: /api/waitlist serverless route (prerender=false) + Resend, with
  dual JSON/303 responses; WaitlistForm progressive enhancement bound on
  astro:page-load (survives ClientRouter swaps); honeypot + validation.
- New components (Icon, Placeholder, DemoVideo, SectionBand, Chip, EECredit,
  StatBar, Pillar, Step, CheckItem, WaitlistForm) + 13 section components.
- Redesigned Header (sticky, mobile menu, CTA) + 4-col Footer (KiCad
  attribution); BaseLayout gains full-bleed mode + OG/canonical meta.
- Assets scaffolded with marked placeholders; EE/generic/client SVGs copied.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

site/public/icons/right-arrow.svg

@@ -0,0 +1,3 @@
+<svg width="8" height="13" viewBox="0 0 8 13" fill="none" xmlns="http://www.w3.org/2000/svg">
+<path d="M4.6 6.5L0 1.9L1.4 0.5L7.4 6.5L1.4 12.5L0 11.1L4.6 6.5Z" fill="white"/>
+</svg>