feat(seo): migrate to build-time prerendering with vite-react-ssg

Fixes SeoHead (title/canonical/hreflang/JSON-LD) to render during SSR via
vite-react-ssg's Head component instead of a client-only useEffect, so
crawlers see real tags without executing JS. Also fixes an SSR crash in
useTheme (localStorage access with no window during server render) and
removes now-duplicated static meta tags from index.html.

frontend/vite.config.js

@@ -1,5 +1,20 @@
 import { defineConfig } from 'vite';
 import react from '@vitejs/plugin-react';
+import fs from 'node:fs';
+import path from 'node:path';
+import { listAllRoutePaths } from './src/data/routePaths.js';
+
+const SITE_URL = 'https://convert.ombrora.com';
+
+function writeSitemap(outDir) {
+  const paths = listAllRoutePaths();
+  const lastmod = new Date().toISOString().slice(0, 10);
+  const urls = paths
+    .map((p) => `  <url>\n    <loc>${SITE_URL}${p}/</loc>\n    <lastmod>${lastmod}</lastmod>\n  </url>`)
+    .join('\n');
+  const sitemap = `<?xml version="1.0" encoding="UTF-8"?>\n<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">\n${urls}\n</urlset>\n`;
+  fs.writeFileSync(path.join(outDir, 'sitemap.xml'), sitemap);
+}
 
 export default defineConfig({
   plugins: [react()],
@@ -8,4 +23,9 @@ export default defineConfig({
       '/api': 'http://localhost:3000',
     },
   },
+  ssgOptions: {
+    dirStyle: 'nested',
+    script: 'async',
+    onFinished: writeSitemap,
+  },
 });