Update BPM customization

New Features:

• 🎼 BPM Input - Enter any BPM from 30 to 300
• ⚡ Apply Button - Click or press ENTER to apply your custom BPM
• 📊 Live BPM Display - Shows the current BPM being used
• 🔄 Auto-conversion - BPM is automatically converted to beat intervals
• 🎯 Smart Switching - Selecting a difficulty preset updates the BPM, or use custom BPM to override