feat: add reset functionality for timer

[snkhn007]

This PR adds a Reset button to the productivity timer.]

Changes Made

• Added a Reset button alongside the Start and Stop buttons.
• Implemented a resetTimer() function.
• Timer now resets instantly to 00:00:00.
• Timer automatically stops when reset.
• Added initialization to ensure the timer displays 00:00:00 correctly.

Testing

• Started the timer and verified it increments correctly.
• Clicked Reset and verified the timer resets to 00:00:00.
• Verified the timer stops automatically after reset.
• Verified the timer can be started again after resetting.

Fixes #115