foundation: resolve platform in injection context for dot background teardown

Co-authored-by: Cursor <cursoragent@cursor.com>

AGENTS.md

@@ -100,6 +100,7 @@ German and English are written idiomatically per language, never machine-transla
 - Use the injected `DOCUMENT` token when the document element is required (it works on server and browser)
 - No user-agent sniffing; use CSS media queries for device capability
 - Capability helpers live in `src/app/core/platform/browser.ts` and return conservative defaults on the server
+- Helpers that use `inject()` must be called from a field initializer or constructor, never from a lifecycle hook or callback, and the resolved value must be stored on the instance
 - Every addressable route must remain prerenderable
 
 ## Accessibility checklist

src/app/components/dot-background/dot-background.spec.ts

@@ -25,4 +25,29 @@ describe('DotBackground', () => {
   it('should create', () => {
     expect(component).toBeTruthy();
   });
+
+  it('does not throw when destroyed after browser initialization', async () => {
+    const gradient = { addColorStop: vi.fn() };
+    const context = {
+      clearRect: vi.fn(),
+      beginPath: vi.fn(),
+      arc: vi.fn(),
+      fill: vi.fn(),
+      createRadialGradient: vi.fn(() => gradient),
+    };
+
+    vi.spyOn(HTMLCanvasElement.prototype, 'getContext').mockReturnValue(
+      context as unknown as CanvasRenderingContext2D,
+    );
+    const animationFrameSpy = vi.spyOn(window, 'requestAnimationFrame').mockReturnValue(0);
+
+    const initializedFixture = TestBed.createComponent(DotBackground);
+    initializedFixture.detectChanges();
+    await initializedFixture.whenStable();
+
+    expect(() => initializedFixture.destroy()).not.toThrow();
+
+    animationFrameSpy.mockRestore();
+    vi.restoreAllMocks();
+  });
 });

src/app/components/dot-background/dot-background.ts

@@ -21,6 +21,7 @@ export class DotBackground implements OnDestroy {
 
   private readonly ngZone = inject(NgZone);
   private readonly coarsePointer = prefersCoarsePointer();
+  private readonly isBrowser = isBrowserPlatform();
 
   private ctx: CanvasRenderingContext2D | undefined;
   private dots: Dot[] = [];
@@ -50,7 +51,7 @@ export class DotBackground implements OnDestroy {
 
     cancelAnimationFrame(this.animationId);
 
-    if (isBrowserPlatform()) {
+    if (this.isBrowser) {
       window.removeEventListener('resize', this.resize);
       window.removeEventListener('mousemove', this.onMouseMove);
       window.removeEventListener('click', this.onMouseClick);