test(time): verify getSecondsUntilUTCMidnight stays positive (JhaSourav07#776)

## Description
Added a property-based style test to verify that
`getSecondsUntilUTCMidnight()` always returns a positive value for every
hour of the day.

Fixes JhaSourav07#731 

## Pillar

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

## Visual Preview
N/A

## 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.
- [ ] 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

utils/time.test.ts

@@ -65,6 +65,19 @@ describe('getSecondsUntilUTCMidnight', () => {
   });
 });
 
+it('returns positive seconds for every hour of day', () => {
+  for (let hour = 0; hour < 24; hour++) {
+    const fakeDate = new Date(Date.UTC(2025, 0, 1, hour, 0, 0));
+
+    vi.setSystemTime(fakeDate);
+
+    const seconds = getSecondsUntilUTCMidnight();
+
+    expect(seconds).toBeGreaterThan(0);
+    expect(seconds).toBeLessThanOrEqual(86400);
+  }
+});
+
 describe('getSecondsUntilMidnightInTimezone', () => {
   beforeEach(() => {
     vi.useFakeTimers();