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/components/StatBar.astro

@@ -0,0 +1,48 @@
+---
+/** StatBar — thin band of mono numerals (§8). */
+interface Props {
+  stats: { value: string; label: string }[];
+}
+const { stats } = Astro.props;
+---
+
+<dl class="statbar">
+  {
+    stats.map((s) => (
+      <div class="stat">
+        <dt class="stat-value">{s.value}</dt>
+        <dd class="stat-label muted">{s.label}</dd>
+      </div>
+    ))
+  }
+</dl>
+
+<style>
+  .statbar {
+    display: grid;
+    grid-template-columns: repeat(4, 1fr);
+    gap: 1.5rem;
+    margin: 0;
+    text-align: center;
+  }
+  .stat {
+    display: flex;
+    flex-direction: column;
+    gap: 0.35rem;
+  }
+  .stat-value {
+    font-family: var(--font-mono);
+    font-size: clamp(1.2rem, 2.4vw, 1.6rem);
+    font-weight: 700;
+    color: var(--fg);
+  }
+  .stat-label {
+    margin: 0;
+    font-size: 0.85rem;
+  }
+  @media (max-width: 680px) {
+    .statbar {
+      grid-template-columns: 1fr 1fr;
+    }
+  }
+</style>