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/app/structured-data/structured-data.component.spec.ts

@@ -0,0 +1,49 @@
+// Load the JIT compiler BEFORE any Angular router/common imports
+import '@angular/compiler';
+
+import { describe, it, expect, beforeEach, vi } from 'vitest';
+import { Injector, PLATFORM_ID } from '@angular/core';
+import { StructuredDataComponent } from './structured-data.component';
+import { StructuredDataService } from '../services/structured-data.service';
+
+describe('StructuredDataComponent — SSR compatibility', () => {
+  let serviceMock: any;
+
+  beforeEach(() => {
+    serviceMock = {
+      structuredData: vi.fn(() => []),
+    };
+  });
+
+  it('should NOT throw when instantiated on the server platform', () => {
+    expect(() => {
+      Injector.create({
+        providers: [
+          { provide: PLATFORM_ID, useValue: 'server' },
+          { provide: StructuredDataService, useValue: serviceMock },
+          {
+            provide: StructuredDataComponent,
+            useFactory: () => new StructuredDataComponent(),
+            deps: [],
+          },
+        ],
+      });
+    }).not.toThrow();
+  });
+
+  it('should instantiate without errors on the browser platform', () => {
+    expect(() => {
+      Injector.create({
+        providers: [
+          { provide: PLATFORM_ID, useValue: 'browser' },
+          { provide: StructuredDataService, useValue: serviceMock },
+          {
+            provide: StructuredDataComponent,
+            useFactory: () => new StructuredDataComponent(),
+            deps: [],
+          },
+        ],
+      });
+    }).not.toThrow();
+  });
+});