feat: add design system, theme toggle, SEO head and base layout

- Huemint Magazine tokens for light/dark themes with AA-safe accent variants
- FOUC-free theme init + toggle (localStorage, follows OS until chosen)
- Seo component (canonical, OG, Twitter, WebSite/Person JSON-LD)
- Header with popover mobile menu, 4-column footer with social links
- Container, GridLines, SectionHeading, Button, Badge, Card, ExternalLink
- site.json / social.json validated with zod
- dev-only /styleguide preview

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

src/lib/data.ts

@@ -0,0 +1,67 @@
+/**
+ * Typed, validated access to src/data/*.json.
+ * A schema mismatch throws at import time, so `astro build` fails loudly.
+ */
+import { z } from 'astro/zod';
+import siteJson from '../data/site.json';
+import socialJson from '../data/social.json';
+
+const link = z.object({ label: z.string().min(1), href: z.string().min(1) });
+
+const siteSchema = z.object({
+  name: z.string(),
+  title: z.string(),
+  description: z.string().max(160),
+  url: z.url(),
+  lang: z.string(),
+  locale: z.string(),
+  titleSeparator: z.string(),
+  author: z.object({ name: z.string(), callsign: z.string(), locator: z.string() }),
+  postsPerPage: z.number().int().positive(),
+  nav: z.array(link).min(1),
+  footer: z.object({
+    about: z.string(),
+    quickLinksTitle: z.string(),
+    legalTitle: z.string(),
+    socialTitle: z.string(),
+    rss: link,
+    legal: z.array(link),
+    license: z.string(),
+    signoff: z.string(),
+  }),
+  ui: z.object({
+    skipToContent: z.string(),
+    mainNav: z.string(),
+    footerNav: z.string(),
+    menuOpen: z.string(),
+    menuClose: z.string(),
+    themeToggle: z.string(),
+    newTab: z.string(),
+    home: z.string(),
+  }),
+});
+
+const socialSchema = z.array(
+  z.object({
+    id: z.string(),
+    label: z.string(),
+    // Empty string = account not set → hidden in the UI.
+    href: z.union([z.url(), z.literal('')]),
+    icon: z.string().startsWith('tabler:'),
+    handle: z.string().optional(),
+  }),
+);
+
+function parse<T>(name: string, schema: z.ZodType<T>, data: unknown): T {
+  const result = schema.safeParse(data);
+  if (!result.success) {
+    throw new Error(`src/data/${name} is invalid:\n${z.prettifyError(result.error)}`);
+  }
+  return result.data;
+}
+
+export const site = parse('site.json', siteSchema, siteJson);
+export const social = parse('social.json', socialSchema, socialJson).filter((s) => s.href);
+
+export type Site = z.infer<typeof siteSchema>;
+export type SocialLink = (typeof social)[number];