test(calculate): verify streak calculations across leap and non-leap years (JhaSourav07#1777)

## Description

Fixes JhaSourav07#1471 

## Pillar

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

## Visual Preview

N/A — Test suite addition. No user interface or SVG layout modifications
were made.

## 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 starred 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] (Recommended) I joined the CommitPulse Discord community for
contributor discussions, mentorship, and faster PR support.

Author: JhaSourav07

lib/calculate.test.ts

@@ -270,6 +270,84 @@ describe('calculateStreak', () => {
     expect(result.longestStreak).toBe(10);
     expect(result.currentStreak).toBe(5);
   });
+
+  it('correctly handles leap years and non-leap years during the Feb 28 to Mar 1 transition', () => {
+    // Helper to construct a ContributionCalendar with explicit dates
+    const buildCustomCalendar = (
+      daysData: { date: string; count: number }[]
+    ): ContributionCalendar => {
+      const weeks = [];
+      for (let i = 0; i < daysData.length; i += 7) {
+        const slice = daysData.slice(i, i + 7);
+        weeks.push({
+          contributionDays: slice.map((day) => ({
+            contributionCount: day.count,
+            date: day.date,
+          })),
+        });
+      }
+      return {
+        totalContributions: daysData.reduce((sum, d) => sum + d.count, 0),
+        weeks,
+      };
+    };
+
+    // --- Case 1: Non-Leap Year (2023) ---
+    // In 2023, Feb has 28 days. Feb 28 is followed directly by Mar 1.
+    const nonLeapCalendar = buildCustomCalendar([
+      { date: '2023-02-27', count: 1 },
+      { date: '2023-02-28', count: 1 },
+      { date: '2023-03-01', count: 1 },
+      { date: '2023-03-02', count: 1 },
+    ]);
+
+    // Evaluating on March 2, 2023:
+    // With commits on Feb 27, Feb 28, Mar 1, and Mar 2, the streak should be continuous (4 days).
+    const resultNonLeap = calculateStreak(nonLeapCalendar, 'UTC', new Date('2023-03-02T12:00:00Z'));
+    expect(nonLeapCalendar.totalContributions).toBe(4);
+    expect(resultNonLeap.currentStreak).toBe(4);
+    expect(resultNonLeap.longestStreak).toBe(4);
+
+    // --- Case 2: Leap Year (2024) ---
+    // In 2024, Feb has 29 days.
+    // If they commit on Feb 28, Feb 29, and Mar 1: streak should be 3.
+    const leapCalendarContinuous = buildCustomCalendar([
+      { date: '2024-02-27', count: 0 },
+      { date: '2024-02-28', count: 1 },
+      { date: '2024-02-29', count: 1 },
+      { date: '2024-03-01', count: 1 },
+    ]);
+
+    const resultLeapContinuous = calculateStreak(
+      leapCalendarContinuous,
+      'UTC',
+      new Date('2024-03-01T12:00:00Z')
+    );
+    expect(resultLeapContinuous.currentStreak).toBe(3);
+    expect(resultLeapContinuous.longestStreak).toBe(3);
+
+    // --- Case 3: Leap Year (2024) with a gap on Feb 29 ---
+    // In 2024, if they commit on Feb 28 and Mar 1 but miss Feb 29:
+    // Evaluating on Mar 1 (grace period = 1):
+    // Today (Mar 1) has 1 commit. Yesterday (Feb 29) has 0 commits.
+    // Since grace is 1, the streak is alive.
+    // However, since Feb 29 is 0, the backward count stops after today (Mar 1).
+    // So the current streak should be 1, and the longest streak should be 1.
+    const leapCalendarWithGap = buildCustomCalendar([
+      { date: '2024-02-27', count: 0 },
+      { date: '2024-02-28', count: 1 },
+      { date: '2024-02-29', count: 0 }, // Gap on leap day!
+      { date: '2024-03-01', count: 1 },
+    ]);
+
+    const resultLeapGap = calculateStreak(
+      leapCalendarWithGap,
+      'UTC',
+      new Date('2024-03-01T12:00:00Z')
+    );
+    expect(resultLeapGap.currentStreak).toBe(1);
+    expect(resultLeapGap.longestStreak).toBe(1);
+  });
 });
 it('handles massive single-day commit spike timeline', () => {
   const calendar = buildCalendar([