test: add theme contrast tests for ScrollRestoration (JhaSourav07#3179)

## Description

Fixes JhaSourav07#2825

Adds theme contrast test coverage for the ScrollRestoration component by
verifying consistent behavior in both dark and light theme environments.
The tests validate rendering, scroll restoration, and scroll position
persistence across theme modes.

## Pillar

* [ ] 🎨 Pillar 1 — New Theme Design
* [ ] 📐 Pillar 2 — Geometric SVG Improvement
* [ ] 🕐 Pillar 3 — Timezone Logic Optimization
* [x] 🛠️ Other (Bug fix, refactoring, docs)

## Visual Preview

N/A (test-only change)

## Checklist before requesting a review:

* [x] I have read the `CONTRIBUTING.md` file.
* [ ] I have tested these changes locally
(`localhost:3000/api/streak?user=YOUR_USERNAME`).
* [x] I have run `npm run format` and `npm run lint` locally and
resolved all errors (CI will fail otherwise).
* [x] My commits follow the Conventional Commits format (e.g.,
`feat(themes): ...`, `fix(calculate): ...`).
* [ ] I have updated `README.md` if I added a new theme or URL
parameter.
* [x] I have started the repo.
* [x] I have made sure that i have only one commit to merge in this PR.
* [ ] The SVG output matches the CommitPulse "premium quality" aesthetic
standard (no raw elements, smooth animations, correct fonts).
* [x] (Recommended) I joined the CommitPulse Discord community for
contributor discussions, mentorship, and faster PR support.

Author: JhaSourav07

app/components/ScrollRestoration.theme-contrast.test.tsx

@@ -0,0 +1,81 @@
+import { render } from '@testing-library/react';
+import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
+
+import ScrollRestoration from './ScrollRestoration';
+
+const mockUsePathname = vi.fn();
+const originalScrollY = window.scrollY;
+
+vi.mock('next/navigation', () => ({
+  usePathname: () => mockUsePathname(),
+}));
+
+describe('ScrollRestoration theme contrast behavior', () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+    sessionStorage.clear();
+
+    document.documentElement.classList.remove('dark');
+
+    mockUsePathname.mockReturnValue('/');
+
+    window.scrollTo = vi.fn();
+  });
+  afterEach(() => {
+    Object.defineProperty(window, 'scrollY', {
+      value: originalScrollY,
+      configurable: true,
+    });
+  });
+
+  it('renders safely in dark theme mode', () => {
+    document.documentElement.classList.add('dark');
+
+    const { container } = render(<ScrollRestoration />);
+
+    expect(container.firstChild).toBeNull();
+  });
+
+  it('renders safely in light theme mode', () => {
+    document.documentElement.classList.remove('dark');
+
+    const { container } = render(<ScrollRestoration />);
+
+    expect(container.firstChild).toBeNull();
+  });
+
+  it('restores saved scroll position in dark theme mode', () => {
+    document.documentElement.classList.add('dark');
+
+    sessionStorage.setItem('scroll-position-/', '400');
+
+    render(<ScrollRestoration />);
+
+    expect(window.scrollTo).toHaveBeenCalledWith(0, 400);
+  });
+
+  it('restores saved scroll position in light theme mode', () => {
+    document.documentElement.classList.remove('dark');
+
+    sessionStorage.setItem('scroll-position-/', '200');
+
+    render(<ScrollRestoration />);
+
+    expect(window.scrollTo).toHaveBeenCalledWith(0, 200);
+  });
+
+  it('stores scroll position consistently regardless of theme', () => {
+    document.documentElement.classList.add('dark');
+
+    Object.defineProperty(window, 'scrollY', {
+      value: 500,
+      configurable: true,
+    });
+
+    render(<ScrollRestoration />);
+
+    window.dispatchEvent(new Event('scroll'));
+
+    expect(sessionStorage.getItem('scroll-position-/')).toBe('500');
+  });
+});