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.

package.json

@@ -49,7 +49,8 @@
     "sharp": "^0.35.3",
     "tar": "^7.5.22",
     "turndown": "^7.2.4",
-    "uuid": "^14.0.1"
+    "uuid": "^14.0.1",
+    "vite-react-ssg": "^0.9.2"
   },
   "devDependencies": {
     "@vitejs/plugin-react": "^6.0.5",