This commit introduces a new Flak Cannon tower type with:
- Complete tower configuration including cost, damage stats, and range for all 3 levels
- Visual upgrades with rotating mid-sections for level 2/3
- Sound effects (flak.mp3) for firing and explosions
- AOE shot mechanics that fire multiple projectiles in a spread pattern
- Rotation handling to prevent conflicts with tower aiming logic
The flak cannon uses a unique firing pattern where it shoots multiple projectiles in different directions within its AOE radius, creating an explosion effect. The upgrade system adds visual elements like rotating mid-sections and changes sprite textures for each level.
