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/mail.svg

@@ -0,0 +1,3 @@
+<svg width="20" height="17" viewBox="0 0 20 17" fill="none" xmlns="http://www.w3.org/2000/svg">
+<path d="M2 16.6445C1.45 16.6445 0.979167 16.4487 0.5875 16.057C0.195833 15.6654 0 15.1945 0 14.6445V2.64453C0 2.09453 0.195833 1.6237 0.5875 1.23203C0.979167 0.840365 1.45 0.644531 2 0.644531H18C18.55 0.644531 19.0208 0.840365 19.4125 1.23203C19.8042 1.6237 20 2.09453 20 2.64453V14.6445C20 15.1945 19.8042 15.6654 19.4125 16.057C19.0208 16.4487 18.55 16.6445 18 16.6445H2ZM10 9.64453L2 4.64453V14.6445H18V4.64453L10 9.64453ZM10 7.64453L18 2.64453H2L10 7.64453ZM2 4.64453V2.64453V14.6445V4.64453Z" fill="#A9A9A9"/>
+</svg>