test: add timezone boundary alignment coverage (JhaSourav07#3247)

## Description

Fixes JhaSourav07#2800 

## Pillar

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

## Visual Preview

## 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.
- [x] I have started the repo.
- [x] I have made sure that i have only one commit to merge in this PR.
- [x] The SVG output matches the CommitPulse "premium quality" aesthetic
standard (no raw elements, smooth animations, correct fonts).
- [x] I joined the CommitPulse Discord community for contributor
discussions, mentorship, and faster PR support.

Author: JhaSourav07

lib/calculate.test.ts

@@ -1320,6 +1320,30 @@ describe('calculateStreak — timezone awareness', () => {
     const result = calculateStreak(tzCalendar, 'UTC', nowUTC);
     expect(result.todayDate).toBe('2024-06-16');
   });
+
+  it('handles timezone boundary alignment between UTC, IST and JST', () => {
+    const calendar = {
+      totalContributions: 2,
+      weeks: [
+        {
+          contributionDays: [
+            { contributionCount: 1, date: '2024-06-14' },
+            { contributionCount: 1, date: '2024-06-15' },
+          ],
+        },
+      ],
+    };
+
+    const now = new Date('2024-06-14T20:00:00Z');
+
+    const utcResult = calculateStreak(calendar, 'UTC', now);
+    const istResult = calculateStreak(calendar, 'Asia/Kolkata', now);
+    const jstResult = calculateStreak(calendar, 'Asia/Tokyo', now);
+
+    expect(utcResult.todayDate).toBe('2024-06-14');
+    expect(istResult.todayDate).toBe('2024-06-15');
+    expect(jstResult.todayDate).toBe('2024-06-15');
+  });
 });
 
 describe('isStreakAlive', () => {