Implement game timer with best time tracking

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