feat: add SSR and prerendering support

- Install @angular/ssr and @angular/platform-server (v21.2.x)
- Create server entry point (server.ts) and server bootstrap (main.server.ts)
- Add server routes config (app.server.routes.ts) with 5 prerendered routes:
  /, /about, /schedule, /events, /contact
- Update angular.json with server/ssr/prerender build configuration
- Add prerender-routes.txt for build-time static route generation
- Fix SSR compatibility:
  - App component: guard DOM access with isPlatformBrowser
  - StructuredDataComponent: skip head manipulation on server
  - ThemeService: skip CSS variable application on server
  - ShowService/EventService: return empty arrays on API failure
    (graceful fallback during prerender with no backend)
- Prerendered output verified: 5 static HTML routes in dist/

src/app/app.server.routes.ts

@@ -0,0 +1,19 @@
+import { RenderMode, ServerRoute } from '@angular/ssr';
+
+/**
+ * Server routes configuration for Angular SSR.
+ *
+ * The five public routes are prerendered at build time (SSG) for optimal SEO.
+ * Authenticated routes (/login, /admin) use SSR for dynamic rendering.
+ * All other routes fall back to SSR.
+ */
+export const serverRoutes: ServerRoute[] = [
+  { path: '', renderMode: RenderMode.Prerender },
+  { path: 'about', renderMode: RenderMode.Prerender },
+  { path: 'schedule', renderMode: RenderMode.Prerender },
+  { path: 'events', renderMode: RenderMode.Prerender },
+  { path: 'contact', renderMode: RenderMode.Prerender },
+  { path: 'login', renderMode: RenderMode.Server },
+  { path: 'admin', renderMode: RenderMode.Server },
+  { path: '**', renderMode: RenderMode.Server },
+];