feat: core layout, UI atoms, homepage + behaviour modules

- UI atoms: Chip, Button, SplitCvButton, LangSwitch, ThemeToggle, Card,
  SectionLabel, TraceMotif, FramedImage
- core: Base/Seo/Header/MobileNav/Footer/HintBar/SkipLink/ThemeScript
- homepage sections: Hero, ProofStrip, SkillsGrid, ProjectCards,
  ExperienceTimeline, AboutTeaser, ContactBand + Homepage composition (EN/NO)
- behaviour modules: theme, nav (overlay/hint/copy/header), observer (reveal/spy),
  lightbox, form — progressive enhancement, fail-silent
- SEO head component: meta, hreflang from slug map, JSON-LD, OG references
- move content data to src/data (avoid Astro reserved content-collection folder)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

site/src/scripts/theme.ts

@@ -0,0 +1,28 @@
+/*
+  Theme toggle (A6). The no-flash init in ThemeScript already set data-theme before
+  paint; this only wires the button, persists the choice, and keeps the aria-label
+  describing the *next* action. Fail-silent if the button is absent.
+*/
+const html = document.documentElement;
+const btn = document.getElementById('theme-toggle');
+
+function labelFor(theme: string): string {
+  if (!btn) return '';
+  return theme === 'dark'
+    ? (btn.dataset.toLight ?? '')
+    : (btn.dataset.toDark ?? '');
+}
+
+if (btn) {
+  btn.setAttribute('aria-label', labelFor(html.dataset.theme ?? 'dark'));
+  btn.addEventListener('click', () => {
+    const next = html.dataset.theme === 'light' ? 'dark' : 'light';
+    html.dataset.theme = next;
+    try {
+      localStorage.setItem('theme', next);
+    } catch {
+      /* private mode — ignore */
+    }
+    btn.setAttribute('aria-label', labelFor(next));
+  });
+}