test(utils): check boundary robustness of time normalization (Variation 2) (JhaSourav07#1877)

## Description
Adds a new test block verifying the time normalization utility converts
timestamps cleanly to target timezone offsets without calendar shifting.

Closes JhaSourav07#1559

## 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 -- no SVG changes.

## 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.
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standard (no raw elements, smooth animations, correct fonts).
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Author: JhaSourav07

utils/time.test.ts

@@ -233,3 +233,53 @@ describe('getSecondsUntilUTCMidnight — sliding window boundary robustness', ()
     }
   });
 });
+
+describe('getSecondsUntilMidnightInTimezone — extreme timezone offset boundary robustness', () => {
+  beforeEach(() => {
+    vi.useFakeTimers();
+  });
+
+  afterEach(() => {
+    vi.useRealTimers();
+  });
+
+  it('converts timestamp cleanly to target offset without calendar shifting (UTC+5:30)', () => {
+    // UTC 2024-06-15T18:30:00Z = 2024-06-16T00:00:00 in Asia/Kolkata (UTC+5:30)
+    // So exactly at local midnight → 86400 seconds remaining
+    vi.setSystemTime(new Date('2024-06-15T18:30:00.000Z'));
+
+    expect(getSecondsUntilMidnightInTimezone('Asia/Kolkata')).toBe(86400);
+  });
+
+  it('converts timestamp cleanly to target offset without calendar shifting (UTC+14)', () => {
+    // UTC 2024-06-14T10:00:00Z = 2024-06-15T00:00:00 in Pacific/Kiritimati (UTC+14)
+    // Exactly local midnight → 86400 seconds remaining, no date shift
+    vi.setSystemTime(new Date('2024-06-14T10:00:00.000Z'));
+
+    expect(getSecondsUntilMidnightInTimezone('Pacific/Kiritimati')).toBe(86400);
+  });
+
+  it('converts timestamp cleanly to target offset without calendar shifting (UTC-12)', () => {
+    // UTC 2024-06-15T12:00:00Z = 2024-06-15T00:00:00 in Etc/GMT+12 (UTC-12)
+    // Exactly local midnight → 86400 seconds remaining, no date shift
+    vi.setSystemTime(new Date('2024-06-15T12:00:00.000Z'));
+
+    expect(getSecondsUntilMidnightInTimezone('Etc/GMT+12')).toBe(86400);
+  });
+
+  it('handles a timestamp near year-end boundary without calendar shifting (UTC+14)', () => {
+    // UTC 2023-12-31T10:00:00Z = 2024-01-01T00:00:00 in Pacific/Kiritimati
+    // Crosses year boundary cleanly → 86400 seconds remaining
+    vi.setSystemTime(new Date('2023-12-31T10:00:00.000Z'));
+
+    expect(getSecondsUntilMidnightInTimezone('Pacific/Kiritimati')).toBe(86400);
+  });
+
+  it('handles a timestamp near year-end boundary without calendar shifting (UTC-11)', () => {
+    // UTC 2024-01-01T11:00:00Z = 2024-01-01T00:00:00 in Pacific/Midway (UTC-11)
+    // Crosses year boundary cleanly → 86400 seconds remaining
+    vi.setSystemTime(new Date('2024-01-01T11:00:00.000Z'));
+
+    expect(getSecondsUntilMidnightInTimezone('Pacific/Midway')).toBe(86400);
+  });
+});