feat(seo): site config, sitemap, robots.txt, llms.txt and a real 404 page

- astro.config: site, trailingSlash "never", build.format "file".
- /sitemap-index.xml + /sitemap-0.xml as prerendered endpoints (no new
  dependency); pages are derived from src/pages/*.astro, 404 excluded.
- robots.txt: all bots allowed, AI crawlers/training bots explicitly
  allowed (Linus's decision), /download/ disallowed, sitemap line.
- /llms.txt generated from business.ts, incl. usage-with-attribution note.
- 404.astro in Swedish with links back, noindex.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WvE3C43Uwtv2vtnoFBLDw7

src/pages/404.astro

@@ -0,0 +1,35 @@
+---
+// src/pages/404.astro -> dist/404.html (servern svarar med status 404)
+import MainLayout from '../layouts/MainLayout.astro';
+import WaveBackground from '../components/WaveBackground.astro';
+
+const links = [
+  { href: '/', label: 'Startsidan' },
+  { href: '/portfolio', label: 'Portfolio' },
+  { href: '/tjanster', label: 'Tjänster och bokning' },
+  { href: '/contact', label: 'Kontakt' },
+];
+---
+
+<MainLayout
+  title="Sidan hittades inte | Images by Olofsson"
+  description="Sidan du letade efter finns inte. Gå till startsidan, portfolion eller tjänsterna hos fotograf Linus Olofsson."
+  noindex
+>
+  <WaveBackground opacity="opacity-60" />
+
+  <div class="container mx-auto px-4 py-20 md:py-32 relative z-10 text-center">
+    <p class="text-sm text-gray-400 uppercase tracking-widest mb-4">Fel 404</p>
+    <h1 class="text-4xl md:text-6xl font-bold mb-6">Sidan hittades inte</h1>
+    <p class="text-gray-300 text-lg max-w-xl mx-auto mb-10">
+      Sidan du letade efter finns inte längre, eller så har adressen ändrats. Här kan du hitta tillbaka:
+    </p>
+    <ul class="flex flex-wrap justify-center gap-4">
+      {links.map((l) => (
+        <li>
+          <a href={l.href} class="inline-block px-6 py-3 rounded-full border border-white/20 text-white hover:bg-white hover:text-black transition-colors">{l.label}</a>
+        </li>
+      ))}
+    </ul>
+  </div>
+</MainLayout>