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/ButtonLink.tsx

@@ -0,0 +1,31 @@
+import Link from "next/link";
+import * as React from "react";
+
+type ButtonVariant = "primary" | "secondary";
+
+type ButtonLinkProps = {
+  readonly href: string;
+  readonly children: React.ReactNode;
+  readonly variant?: ButtonVariant;
+  readonly className?: string;
+};
+
+const BASE =
+  "inline-flex items-center justify-center rounded-md px-5 py-2.5 text-sm font-semibold transition-colors focus-visible:outline focus-visible:outline-2 focus-visible:outline-offset-2 focus-visible:outline-accent";
+
+const VARIANTS: Record<ButtonVariant, string> = {
+  primary: "bg-accent !text-accent-fg hover:bg-accent-hover",
+  secondary:
+    "border border-border-strong !text-fg hover:border-muted hover:bg-surface",
+};
+
+export const ButtonLink: React.FC<ButtonLinkProps> = ({
+  href,
+  children,
+  variant = "primary",
+  className = "",
+}) => (
+  <Link href={href} className={`${BASE} ${VARIANTS[variant]} ${className}`}>
+    {children}
+  </Link>
+);