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

astro.config.mjs

@@ -2,6 +2,15 @@ import { defineConfig } from 'astro/config';
 import tailwindcss from '@tailwindcss/vite';
 
 export default defineConfig({
+  // Kanonisk adress: canonical, og:url, sitemap och JSON-LD byggs från denna.
+  site: 'https://imagesbyolofsson.se',
+  // En URL per sida: /about (aldrig /about/). Servern 301:ar /about/ -> /about.
+  trailingSlash: 'never',
+  build: {
+    // about.astro -> dist/about.html (inte dist/about/index.html).
+    // nginx/Caddy serverar /about via try_files $uri.html.
+    format: 'file',
+  },
   vite: {
     plugins: [tailwindcss()],
   },