refactor(ci): add vitest coverage threshold gate to CI pipeline (JhaSourav07#469)

## Description
Fixes JhaSourav07#401

## 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 — this is a CI/config change, no SVG output affected.

## 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.
- [ ] 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.

## Test Coverage Results
| Metric     | Coverage | Threshold |
|------------|----------|-----------|
| Statements | 86.45%   | 70% ✅    |
| Branches   | 72.93%   | 70% ✅    |
| Functions  | 85.44%   | 70% ✅    |
| Lines      | 88.85%   | 70% ✅    |

235 tests passing across 17 test files.

Author: JhaSourav07

.github/workflows/ci.yml

@@ -34,7 +34,7 @@ jobs:
         run: npm run typecheck
 
       - name: Run Unit Tests (Vitest)
-        run: npm run test
+        run: npm run test:coverage
 
   build:
     name: Production Build

vitest.config.ts

@@ -7,6 +7,16 @@ export default defineConfig({
     globals: true,
     include: ['**/*.test.ts', '**/*.test.tsx'],
     exclude: ['node_modules', '.next'],
+    coverage: {
+      provider: 'v8',
+      reporter: ['text', 'lcov'],
+      thresholds: {
+        statements: 70,
+        branches: 70,
+        functions: 70,
+        lines: 70,
+      },
+    },
   },
   resolve: {
     alias: {