fix(ssr): apply theme CSS vars during SSR and enable client hydration

- ThemeService: remove isPlatformBrowser guard so CSS custom properties
  are applied during SSR/prerender — server-rendered HTML now carries the
  correct color scheme and gradients (fixes missing colors on test site).

- main.ts: add provideClientHydration() so Angular preserves the server-
  rendered DOM on the client instead of destroying and rebuilding it
  (fixes jarring full-page reload between static and dynamic versions).

- Tests: update theme.service.spec.ts to match new behaviour, expand
  coverage with gradient tests and empty-value handling (9 tests).
  Fix structured-data.component.spec.ts import paths. All 54 tests pass.

server.ts

@@ -6,16 +6,53 @@ import {
 } from '@angular/ssr/node';
 import * as http from 'node:http';
 
+/**
+ * Public routes prerendered at build time (SSG — Static Site Generation).
+ * These five routes are compiled to static HTML during `ng build` and
+ * served directly from disk at runtime for optimal performance and SEO.
+ * The list here mirrors `prerender-routes.txt` and the Prerender entries
+ * in `src/app/app.server.routes.ts`.
+ */
+export const PRERENDERED_ROUTES: readonly string[] = [
+  '/',
+  '/about',
+  '/schedule',
+  '/events',
+  '/contact',
+];
+
 const ngApp = new AngularNodeAppEngine();
 
+/**
+ * Normalise a request pathname and check against prerendered routes.
+ * Handles both bare paths (`/about`) and trailing-slash variants (`/about/`).
+ */
+export function isPrerenderedRoute(url: string): boolean {
+  const pathname = new URL(url, 'http://localhost').pathname;
+  const normalised = pathname.endsWith('/') && pathname !== '/'
+    ? pathname.slice(0, -1)
+    : pathname;
+  return PRERENDERED_ROUTES.includes(normalised);
+}
+
 /**
  * Handle incoming requests using the Angular SSR engine.
- * Prerendered routes are served as static HTML at build time.
- * Dynamic routes fall back to server-side rendering.
+ *
+ * Prerendered routes (/ , /about, /schedule, /events, /contact) are served
+ * as pre-generated static HTML from the build output. All other routes
+ * fall back to server-side rendering (SSR) or the fallback 404 handler.
  */
 export default createNodeRequestHandler(async (req, res, next) => {
   try {
+    const prerendered = isPrerenderedRoute(req.url || '/');
+    if (prerendered) {
+      // Prerendered routes are served as static HTML — AngularNodeAppEngine
+      // reads the build-time manifest and returns the pre-generated response.
+      console.log(`[SSR] Prerendered route: ${req.url}`);
+    }
+
     const response = await ngApp.handle(req);
+
     if (response) {
       await writeResponseToNodeResponse(response, res);
     } else {