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.

src/app/constants/RouterConstants.ts

@@ -70,6 +70,11 @@
     LINK: '/imprint',
   };
 
+  static readonly PRIVACY = {
+    PATH: 'privacy',
+    LINK: '/privacy',
+  };
+
   static readonly STOPWATCH = {
     PATH: 'stopwatch',
     LINK: '/stopwatch',