feat(app): add prerender, self-hosted fonts and privacy page

Fonts come from npm, not Google, so no third-party font requests.
Pages are prerendered at build time; browser-only code is guarded.
nginx serves per-route HTML, a real 404 and gzip.

lighthouserc.json

@@ -2,6 +2,7 @@
   "ci": {
     "collect": {
       "numberOfRuns": 1,
+      "autodiscoverUrlBlocklist": ["/index.csr.html"],
       "staticDistDir": "./dist/playground-frontend/browser",
       "settings": {
         "chromeFlags": "--no-sandbox --headless --disable-gpu --disable-dev-shm-usage"