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/src/components/site/Container.tsx

@@ -0,0 +1,15 @@
+import * as React from "react";
+
+type ContainerProps = {
+  readonly children: React.ReactNode;
+  readonly className?: string;
+};
+
+export const Container: React.FC<ContainerProps> = ({
+  children,
+  className = "",
+}) => (
+  <div className={`mx-auto w-full max-w-6xl px-5 md:px-8 ${className}`}>
+    {children}
+  </div>
+);