Fix timer button negative countdown bug and enhance example app

[ProjectAJ14]

Problem

The timer button was displaying negative countdown values (e.g., "-5s") due to improper state management in the timer logic. When the countdown reached 0, it would continue decrementing and show negative values instead of stopping.

Solution

• Fix timer state management to prevent negative countdown values
• Add comprehensive test coverage for negative countdown prevention
• Enhance example app with modern Material 3 theming

Changes Needed

• Fix timer counter logic to stop at 0
• Add test case for negative countdown prevention
• Implement Material 3 custom theme in example app
• Restructure example app directory for better organization

Impact

This ensures the timer button behaves correctly and provides a better user experience with no confusing negative countdown values.