test(leaderboard): add theme contrast coverage (JhaSourav07#3244)

## Description

Fixes JhaSourav07#2755

Added theme contrast test coverage for the Leaderboard component.

### Changes Made

* Created `components/Leaderboard.theme-contrast.test.tsx`
* Added tests to verify leaderboard renders correctly across themes
* Added checks for dark mode background styling
* Added checks for dark mode text contrast classes
* Added checks for dark mode border contrast classes
* Added checks for hover contrast styling across light and dark themes

## 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 coverage only)

## Checklist before requesting a review:

* [x] I have read the `CONTRIBUTING.md` file.
* [x] 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.
* [x] The SVG output matches the CommitPulse "premium quality" aesthetic
standard (no raw elements, smooth animations, correct fonts).
* [ ] (Recommended) I joined the CommitPulse Discord community for
contributor discussions, mentorship, and faster PR support.

Author: JhaSourav07

components/Leaderboard.theme-contrast.test.tsx

@@ -0,0 +1,83 @@
+import { render } from '@testing-library/react';
+import { describe, expect, it, vi } from 'vitest';
+import type { HTMLAttributes, ReactNode } from 'react';
+
+import Leaderboard from './Leaderboard';
+
+vi.mock('next/image', () => ({
+  default: () => null,
+}));
+
+vi.mock('framer-motion', () => ({
+  motion: {
+    div: ({
+      children,
+      ...props
+    }: HTMLAttributes<HTMLDivElement> & {
+      children?: ReactNode;
+    }) => <div {...props}>{children}</div>,
+  },
+}));
+
+describe('Leaderboard theme contrast behavior', () => {
+  const contributors = [
+    {
+      id: 1,
+      login: 'alice',
+      avatar_url: 'https://example.com/alice.png',
+      contributions: 100,
+      html_url: 'https://github.com/alice',
+    },
+    {
+      id: 2,
+      login: 'bob',
+      avatar_url: 'https://example.com/bob.png',
+      contributions: 90,
+      html_url: 'https://github.com/bob',
+    },
+    {
+      id: 3,
+      login: 'charlie',
+      avatar_url: 'https://example.com/charlie.png',
+      contributions: 80,
+      html_url: 'https://github.com/charlie',
+    },
+    {
+      id: 4,
+      login: 'david',
+      avatar_url: 'https://example.com/david.png',
+      contributions: 70,
+      html_url: 'https://github.com/david',
+    },
+  ];
+
+  it('renders leaderboard container', () => {
+    const { container } = render(<Leaderboard contributors={contributors} />);
+
+    expect(container.firstChild).toBeTruthy();
+  });
+
+  it('includes dark mode background classes', () => {
+    const { container } = render(<Leaderboard contributors={contributors} />);
+
+    expect(container.innerHTML).toContain('dark:bg');
+  });
+
+  it('includes dark mode text classes', () => {
+    const { container } = render(<Leaderboard contributors={contributors} />);
+
+    expect(container.innerHTML).toContain('dark:text-white');
+  });
+
+  it('includes dark mode border classes', () => {
+    const { container } = render(<Leaderboard contributors={contributors} />);
+
+    expect(container.innerHTML).toContain('dark:border');
+  });
+
+  it('includes hover contrast styling for both themes', () => {
+    const { container } = render(<Leaderboard contributors={contributors} />);
+
+    expect(container.innerHTML).toContain('dark:hover');
+  });
+});