test(time):asserted integer output for sub-second precision inputs (JhaSourav07#1874)

## Description
- Adds two test cases to `utils/time.test.ts` — one for
`getSecondsUntilUTCMidnight` and one for
`getSecondsUntilMidnightInTimezone`
- Each test sets the clock to `23:59:59.999` (1 ms before midnight) and
asserts `Number.isInteger(result) === true`
- Explicitly verifies that `Math.floor` is applied correctly and
sub-second remainders never leak into the return value

Fixes JhaSourav07#733

## Pillar

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

## 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): ...`).
- [x] I have updated `README.md` if I added a new theme or URL
parameter.
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- [x] I have made sure that i have only one commit to merge in this PR.
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standard (no raw elements, smooth animations, correct fonts).
- [ ] (Recommended) I joined the CommitPulse Discord community for
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Author: JhaSourav07

utils/time.test.ts

@@ -63,6 +63,14 @@ describe('getSecondsUntilUTCMidnight', () => {
 
     expect(getSecondsUntilUTCMidnight()).toBe(64800); // 18 hours = 64800 s
   });
+
+  it('always returns an integer with sub-second precision input', () => {
+    vi.setSystemTime(new Date('2024-06-15T23:59:59.999Z'));
+
+    const result = getSecondsUntilUTCMidnight();
+    expect(Number.isInteger(result)).toBe(true);
+    expect(result).toBe(0);
+  });
 });
 
 it('returns positive seconds for every hour of day', () => {
@@ -145,6 +153,15 @@ describe('getSecondsUntilMidnightInTimezone', () => {
     }
   });
 
+  it('always returns an integer with sub-second precision input', () => {
+    // 2024-06-16T03:59:59.999Z is 23:59:59.999 in Etc/GMT+4 (UTC-4)
+    vi.setSystemTime(new Date('2024-06-16T03:59:59.999Z'));
+
+    const result = getSecondsUntilMidnightInTimezone('Etc/GMT+4');
+    expect(Number.isInteger(result)).toBe(true);
+    expect(result).toBe(1);
+  });
+
   it('handles extreme timezone Etc/GMT-14 (UTC+14)', () => {
     // UTC 00:00 → local time 14:00 in UTC+14
     vi.setSystemTime(new Date('2024-06-15T00:00:00.000Z'));