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/arrow-outward.svg

@@ -0,0 +1,8 @@
+<svg width="24" height="25" viewBox="0 0 24 25" fill="none" xmlns="http://www.w3.org/2000/svg">
+<mask id="mask0_683_2587" style="mask-type:alpha" maskUnits="userSpaceOnUse" x="0" y="0" width="24" height="25">
+<rect y="0.5" width="24" height="24" fill="#D9D9D9"/>
+</mask>
+<g mask="url(#mask0_683_2587)">
+<path d="M6.4 18.5L5 17.1L14.6 7.5H6V5.5H18V17.5H16V8.9L6.4 18.5Z" fill="#FFD644"/>
+</g>
+</svg>