- Add interval management system to fix memory leaks and cleanup bugs
- Implement win condition (survive 3 minutes) with victory screen
- Add pause functionality (ESC/P key) with pause overlay menu
- Add progressive difficulty (enemy speed/spawn rate increase over time)
- Implement enemy variety: Normal, Fast (green), and Tank (larger, 2 HP)
- Add boss fights every 60 seconds with health bar
- Add power-ups: Extra Life, Triple Shot, Shield (10-15s spawn interval)
- Implement Hardcore Mode (1 life, faster enemies, no power-ups)
- Add high score system with localStorage persistence
- Add background music support with volume control
- Add visual feedback (player hit flash, enemy death animation, shield glow)
- Add mobile touch controls (auto-detected on touch devices)
- Improve collision detection using getBoundingClientRect
- Add responsive styles for mobile devices
