fix:#90

Changes Made

• Fixed issue where clicking the Start button multiple times created multiple intervals.
• Prevented the timer from speeding up due to duplicate setInterval() calls.
• Updated timer state management using a running flag.
• Fixed Stop button behavior to properly stop the timer.
• Fixed Reset button behavior to reset the timer and allow restarting.
• Improved timer completion handling by clearing the interval when the countdown reaches zero.

Result

The Pomodoro timer now starts only once, stops correctly, resets correctly, and maintains a consistent countdown speed.