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/src/sections/HowItWorks.astro

@@ -0,0 +1,44 @@
+---
+import SectionBand from '../components/SectionBand.astro';
+import Step from '../components/Step.astro';
+---
+
+<SectionBand id="how-it-works" labelledby="hiw-h2">
+  <div class="section-head">
+    <h2 id="hiw-h2">From link to layout in three moves.</h2>
+  </div>
+  <div class="steps">
+    <Step n={1} title="Open a board." icon="icons/idea.svg">
+      Paste a link, drag in a <code>.kicad_pcb</code>, or pull from GitHub. It opens in seconds —
+      no install, no account wall to <em>look</em>.
+    </Step>
+    <Step n={2} title="Start a jam." icon="icons/prototype.svg">
+      Share the URL. Teammates join with live cursors and presence. Everyone sees the same
+      board update in real time.
+    </Step>
+    <Step n={3} title="Design, route & export — together." icon="icons/product.svg">
+      Schematic capture, PCB layout, DRC/ERC, 3D, BOM, and Gerbers — the full flow, then hand
+      off to any fab. Your call.
+    </Step>
+  </div>
+</SectionBand>
+
+<style>
+  .steps {
+    display: grid;
+    gap: 1.5rem;
+    grid-template-columns: repeat(3, 1fr);
+  }
+  .steps :global(code) {
+    font-family: var(--font-mono);
+    font-size: 0.9em;
+    background: color-mix(in srgb, var(--bg-soft) 70%, transparent);
+    padding: 0.05rem 0.3rem;
+    border-radius: var(--radius-sm);
+  }
+  @media (max-width: 760px) {
+    .steps {
+      grid-template-columns: 1fr;
+    }
+  }
+</style>