Implement game timer with best time tracking

[debugfest]

Add a visual timer that tracks game duration and saves best completion times to enhance competitive gameplay.

Feature Requirements:

• Real-time timer display (MM:SS format)
• Timer starts on first card flip
• Timer stops when game is completed
• Best time tracking per grid size
• Best time persistence using localStorage
• Timer reset on new game
• Pause/resume functionality (optional)

UI Elements:

• Timer display in header/top section
• Best time indicator
• "New Best Time!" celebration message
• Timer formatting: 02:34 or 1:23:45 for longer games