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>

astro.config.mjs

@@ -7,6 +7,20 @@ import sitemap from '@astrojs/sitemap';
 import node from '@astrojs/node';
 import icon from 'astro-icon';
 
+/**
+ * Dev-only routes (style preview); never part of a production build.
+ * @type {import('astro').AstroIntegration}
+ */
+const devRoutes = {
+  name: 'dev-routes',
+  hooks: {
+    'astro:config:setup': ({ command, injectRoute }) => {
+      if (command !== 'dev') return;
+      injectRoute({ pattern: '/styleguide', entrypoint: './src/dev/Styleguide.astro' });
+    },
+  },
+};
+
 // https://astro.build/config
 export default defineConfig({
   site: 'https://ta1sph.info',
@@ -14,11 +28,7 @@ export default defineConfig({
   // Static by default; only routes with `export const prerender = false` (view counter API) run on the server.
   output: 'static',
   adapter: node({ mode: 'standalone' }),
-  integrations: [
-    mdx(),
-    sitemap({ filter: (page) => !page.includes('/_styleguide/') }),
-    icon({ include: { tabler: ['*'] } }),
-  ],
+  integrations: [mdx(), sitemap(), icon({ include: { tabler: ['*'] } }), devRoutes],
   vite: {
     plugins: [tailwindcss()],
   },