[Test] Add Leap-Year Boundary Test Case to Habit Streak Test Suite

• Labels: good first issue, testing, difficulty:beginner, area:core
• Target File: tests/habits-routines.test.ts
• Estimated Time: 15–20 minutes
• Difficulty: Level 3 (Beginner)

Why This Matters

Habit streak calculations must correctly handle February 28 to February 29 transitions during leap years without resetting the streak to zero.

Current Behavior

tests/habits-routines.test.ts validates daily and weekly habit streaks, but lacks an explicit leap-year boundary test case spanning 2028-02-28 to 2028-02-29 and 2028-03-01.

Requested Change

Add a new test case in tests/habits-routines.test.ts verifying that completing a daily habit on Feb 28, Feb 29 (leap day), and Mar 1 calculates a continuous 3-day streak.

Acceptance Criteria

• Test case added in tests/habits-routines.test.ts.
• Test passes cleanly when run via npm run test:file -- tests/habits-routines.test.ts.
• Existing 56 test suites remain passing.

Verification

npm run test:file -- tests/habits-routines.test.ts

Contributor Notes

Follow the existing test patterns in tests/habits-routines.test.ts.