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/components/ui/SplitCvButton.astro

@@ -0,0 +1,63 @@
+---
+/*
+  CV split button (MICRO_INTERACTIONS): primary zone downloads the current-locale CV;
+  the chevron is a native <details> disclosure listing both languages with file sizes
+  — works with no JavaScript. nav.ts enhances it with outside-click / Esc close.
+*/
+import { useDict, interpolate } from '@i18n/t';
+import type { Locale } from '@i18n/locales';
+import { getProfile } from '@lib/content';
+
+interface Props {
+  locale: Locale;
+  compact?: boolean;
+}
+const { locale, compact = false } = Astro.props;
+const d = useDict(locale);
+const p = getProfile(locale);
+const current = p.cv[locale];
+const other = locale === 'en' ? p.cv.no : p.cv.en;
+const otherLabel = locale === 'en' ? d.cv.norsk : d.cv.english;
+const currentLabel = locale === 'en' ? d.cv.english : d.cv.norsk;
+---
+
+<div class="cv-split relative inline-flex items-stretch">
+  <a
+    href={current.path}
+    download
+    class="inline-flex items-center rounded-l-sm bg-accent px-4 py-2 text-small font-semibold text-accent-ink transition-[filter] duration-[--dur-quick] hover:brightness-105"
+  >
+    {d.cv.download}
+    {!compact && <span class="ml-2 font-mono text-mono-label opacity-70">EN/NO</span>}
+  </a>
+  <details class="cv-menu">
+    <summary
+      class="flex cursor-pointer list-none items-center rounded-r-sm border-l border-accent-ink/20 bg-accent px-2 text-accent-ink transition-[filter] duration-[--dur-quick] hover:brightness-105"
+      aria-label={d.cv.download}
+    >
+      <svg width="14" height="14" viewBox="0 0 24 24" fill="none" aria-hidden="true">
+        <path d="M6 9l6 6 6-6" stroke="currentColor" stroke-width="2" stroke-linecap="round"></path>
+      </svg>
+    </summary>
+    <div
+      class="absolute right-0 top-[calc(100%+8px)] z-30 w-64 overflow-hidden rounded-md border border-line-strong bg-surface-2 shadow-[var(--shadow-overlay)]"
+    >
+      <a
+        href={current.path}
+        download
+        class="flex items-center justify-between gap-2 px-4 py-3 text-small text-ink hover:bg-surface-1"
+      >
+        <span><span class="mr-2 font-mono text-mono-label text-accent">{d.lang[locale]}</span>{currentLabel}</span>
+        <span class="mono-meta">{interpolate(d.cv.fileMeta, { size: current.sizeKb })}</span>
+      </a>
+      <a
+        href={other.path}
+        download
+        class="flex items-center justify-between gap-2 border-t border-line px-4 py-3 text-small text-ink-muted hover:bg-surface-1 hover:text-ink"
+      >
+        <span><span class="mr-2 font-mono text-mono-label">{locale === 'en' ? 'NO' : 'EN'}</span>{otherLabel}</span>
+        <span class="mono-meta">{interpolate(d.cv.fileMeta, { size: other.sizeKb })}</span>
+      </a>
+    </div>
+  </details>
+</div>