Countdown Timer Controls Misalignment and Improve Responsiveness

In the Countdown Timer section of the stopwatch app, the input field ("Minutes"), “Start Countdown” button. This makes the UI look uneven, especially on smaller screens.

**Steps to Reproduce:**

Click on the Countdown Timer tab.
Observe that:
- The “**Minutes**” input and “**Start Countdown**” button are slightly off-center.
- Buttons have inconsistent spacing and padding on different screen sizes.

**Expected Behavior**

- All timer controls should be perfectly centered and evenly spaced, both horizontally and vertically.
- Layout should remain consistent and responsive on mobile and desktop.

@avinash201199 Please assign this issue to me