Overview

Implement a working month calendar in App.tsx — the calendar components are provided, so you focus on rendering the correct days, highlighting today, navigating between months, and handling leap years with plain JavaScript date math.

Requirements

• Render the app title "Calendar" and the weekday header (Mon through Sun).
• Show the current month and year in the header (e.g. August 2026).
• Render the correct number of days for the selected month (28, 29, 30, or 31).
• Place the 1st day of the month in the correct weekday column.
• Fill the empty cells with days from the previous and next month so the grid always completes full weeks, dimming those days visually.
• Navigate to the previous month.
• Navigate to the next month.
• Highlight the current day (today) in the grid.
• Change the year correctly when navigating past December or before January.
• Handle leap years correctly (February has 29 days in a leap year and 28 otherwise).

Notes

• Components.tsx is read-only — components, props, and data-testid attributes are already wired. Implement everything in App.tsx.
• Day 0 of a month is the last day of the previous month, so new Date(year, month + 1, 0).getDate() returns the number of days in month (leap years included).
• new Date(year, month, 1).getDay() returns the weekday of the 1st (0 = Sunday). The week starts on Monday, so the leading-day offset is (getDay() + 6) % 7.
• Navigate with new Date(year, month + offset, 1) — JavaScript rolls the year over automatically.
• Use new Date() for the initial month and to flag today's date.

Tests

1. renders the app title
2. renders the seven weekday labels
3. shows the current month and year in the header
4. renders the correct number of days for the current month
5. places the 1st day on the correct weekday
6. fills the grid with days from the previous and next month
7. navigates to the next month
8. navigates to the previous month
9. changes the year when navigating past December
10. changes the year when navigating before January
11. highlights today's date
12. does not highlight any day when viewing another month
13. renders 28 days for February in a non-leap year
14. renders 29 days for February in a leap year
15. renders 30 days for a 30-day month
16. uses five weeks when the month fits in five