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/package.json

@@ -4,8 +4,8 @@
   "version": "0.0.0",
   "type": "module",
   "scripts": {
-    "dev": "vite",
-    "build": "vite build",
+    "dev": "vite-react-ssg dev",
+    "build": "vite-react-ssg build",
     "lint": "oxlint",
     "preview": "vite preview"
   },
@@ -16,7 +16,8 @@
     "react": "^19.2.7",
     "react-dom": "^19.2.7",
     "react-i18next": "^17.0.11",
-    "react-router-dom": "^6.30.4"
+    "react-router-dom": "^6.30.4",
+    "vite-react-ssg": "^0.9.2"
   },
   "devDependencies": {
     "@types/react": "^19.2.17",