Add LanguageChart donut background style test (JhaSourav07#785)

## Description

Added a test for `LanguageChart` to verify the donut chart gradient
logic includes the expected language color and percentage.

Fixes JhaSourav07#738 

## Pillar

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

## Visual Preview

No visual UI changes. 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`).
- [ ] 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).
- [ ] (Recommended) I joined the CommitPulse Discord community for
contributor discussions, mentorship, and faster PR support.

Author: JhaSourav07

components/dashboard/LanguageChart.test.tsx

@@ -1,8 +1,7 @@
 /* eslint-disable @typescript-eslint/no-explicit-any */
 import { render, screen } from '@testing-library/react';
 import { describe, expect, it, vi } from 'vitest';
-import LanguageChart from './LanguageChart';
-import { buildGradientStops } from './LanguageChart';
+import LanguageChart, { buildGradientStops } from './LanguageChart';
 
 vi.mock('framer-motion', () => ({
   motion: {
@@ -44,6 +43,13 @@ describe('LanguageChart', () => {
     expect(screen.getAllByText('TypeScript')).toHaveLength(2);
     expect(screen.getByText('JavaScript')).toBeDefined();
   });
+
+  it('renders donut background with language color and percentage', () => {
+    const result = buildGradientStops([{ name: 'TypeScript', percentage: 100, color: '#3178c6' }]);
+
+    expect(result).toContain('#3178c6');
+    expect(result).toContain('100%');
+  });
 });
 
 describe('buildGradientStops', () => {

components/dashboard/LanguageChart.tsx

@@ -58,6 +58,7 @@ export default function LanguageChart({ languages }: { languages: LanguageData[]
       <div className="relative w-36 h-36 flex items-center justify-center">
         {/* Donut */}
         <motion.div
+          data-testid="donut-chart"
           initial={{ rotate: -90, scale: 0.8, opacity: 0 }}
           animate={{ rotate: 0, scale: 1, opacity: 1 }}
           transition={{ duration: 0.8, type: 'spring', stiffness: 80 }}