feat: dark theme foundation, new header/footer, shared site primitives

Adds dark design tokens, Inter + JetBrains Mono, a header with Notes,
Process, About and a Book a call button, and a footer with the availability
line driven by the tested domain core. Shared primitives live in
src/components/site, site constants in src/content/site.ts. Legacy pages
(blog, legal, contact, tags, technologies) keep a light surface via
LegacyLightSurface. Stub routes for /notes, /process and /book.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

apps/web/app/(site)/notes/page.tsx

@@ -0,0 +1,18 @@
+import type { Metadata } from "next";
+import { Container } from "@/src/components/site/Container";
+import { SectionHeader } from "@/src/components/site/SectionHeader";
+
+export const metadata: Metadata = { title: "Notes" };
+
+export default function Page() {
+  return (
+    <Container className="py-16 md:py-24">
+      <SectionHeader
+        as="h1"
+        kicker="/notes"
+        title="Notes"
+        lead="This page is coming soon."
+      />
+    </Container>
+  );
+}