Add contributors loading theme contrast tests (JhaSourav07#3120)

## Description

Fixes JhaSourav07#2845

Added isolated Vitest coverage for `app/contributors/loading.tsx`,
focusing on dark/light theme visual cohesion, text contrast
expectations, Tailwind theme classes, and foreground visibility.

## Pillar

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

## Visual Preview

Not applicable — 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/contributors/loading.theme-contrast.test.tsx

@@ -0,0 +1,90 @@
+import { render, screen } from '@testing-library/react';
+import { describe, expect, it } from 'vitest';
+import Loading from './loading';
+
+function hasClasses(element: Element | null, classes: string[]) {
+  expect(element).not.toBeNull();
+
+  for (const className of classes) {
+    expect(element!.classList.contains(className)).toBe(true);
+  }
+}
+
+describe('Contributors loading theme contrast', () => {
+  it('renders an accessible loading status', () => {
+    render(<Loading />);
+
+    const status = screen.getByRole('status');
+
+    expect(status.getAttribute('aria-live')).toBe('polite');
+    expect(screen.getByText('Loading the collective...')).toBeTruthy();
+    expect(screen.getByText('Fetching contributor data from GitHub')).toBeTruthy();
+  });
+
+  it('applies cohesive dark visual shell classes', () => {
+    render(<Loading />);
+
+    const page = screen.getByRole('status').parentElement;
+
+    hasClasses(page, [
+      'flex',
+      'min-h-screen',
+      'items-center',
+      'justify-center',
+      'bg-[#050505]',
+      'text-white',
+    ]);
+  });
+
+  it('keeps contributor loading text visually readable', () => {
+    render(<Loading />);
+
+    const primaryText = screen.getByText('Loading the collective...');
+    const secondaryText = screen.getByText('Fetching contributor data from GitHub');
+
+    hasClasses(primaryText, ['text-zinc-400', 'font-light', 'text-lg']);
+    hasClasses(secondaryText, ['text-sm', 'text-zinc-600', 'font-mono']);
+  });
+
+  it('uses premium spinner styling with visible foreground contrast', () => {
+    render(<Loading />);
+
+    const status = screen.getByRole('status');
+    const spinnerWrapper = status.firstElementChild;
+    const spinner = spinnerWrapper?.firstElementChild ?? null;
+
+    hasClasses(status, ['flex', 'flex-col', 'items-center', 'gap-6']);
+    hasClasses(spinnerWrapper, ['relative']);
+    hasClasses(spinner, [
+      'h-16',
+      'w-16',
+      'animate-spin',
+      'rounded-full',
+      'border-2',
+      'border-white/10',
+      'border-t-cyan-400',
+    ]);
+  });
+
+  it('keeps glow overlay behind spinner without clipping foreground content', () => {
+    render(<Loading />);
+
+    const status = screen.getByRole('status');
+    const spinnerWrapper = status.firstElementChild;
+    const glowOverlay = spinnerWrapper?.children.item(1) ?? null;
+
+    hasClasses(glowOverlay, [
+      'absolute',
+      'inset-0',
+      'h-16',
+      'w-16',
+      'rounded-full',
+      'bg-cyan-400/20',
+      'blur-xl',
+      'animate-pulse',
+    ]);
+
+    expect(status.classList.contains('overflow-hidden')).toBe(false);
+    expect(status.classList.contains('text-transparent')).toBe(false);
+  });
+});