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.

tsconfig.app.json

@@ -1,11 +1,17 @@
-/* To learn more about Typescript configuration file: https://www.typescriptlang.org/docs/handbook/tsconfig-json.html. */
-/* To learn more about Angular compiler options: https://angular.dev/reference/configs/angular-compiler-options. */
-{
+/* To learn more about Typescript configuration file: https://typescriptlang.org/docs/handbook/tsconfig-json.html. */
+/* To learn more about Angular compiler options: https://angular.dev/reference/configs/angular-compiler-options. */{
   "extends": "./tsconfig.json",
   "compilerOptions": {
     "outDir": "./out-tsc/app",
-    "types": []
+    "types": [
+      "node"
+    ]
   },
-  "include": ["src/**/*.ts"],
-  "exclude": ["src/**/*.spec.ts"]
+  "include": [
+    "src/**/*.ts",
+    "server.ts"
+  ],
+  "exclude": [
+    "src/**/*.spec.ts"
+  ]
 }