- 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.
