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.

src/main.ts

@@ -1,6 +1,10 @@
 import 'zone.js';
 import { bootstrapApplication } from '@angular/platform-browser';
+import { provideClientHydration } from '@angular/platform-browser';
 import { appConfig } from './app/app.config';
 import { App } from './app/app';
 
-bootstrapApplication(App, appConfig).catch((err) => console.error(err));
+bootstrapApplication(App, {
+  ...appConfig,
+  providers: [...appConfig.providers, provideClientHydration()],
+}).catch((err) => console.error(err));