Add a final boss

[sidkosmas]

Description

Implement a boss battle feature that triggers after the player successfully completes level 10.

Requirements

Boss Spawn Conditions

• Trigger boss spawn after all UFOs are destroyed at level 10
• Player must complete level 10 within the time limit
• Stop the countdown timer when boss battle begins

Boss Characteristics

• Boss sprite should be 5x larger than regular UFOs
• Boss moves freely on both X and Y axes
• Boss must remain within viewport boundaries at all times
• Boss shoots missiles directly downward (straight down trajectory)

Combat Mechanics

• Direct hit from boss missile = instant game over
• Physical collision between boss and player spaceship = instant game over
• 20 hits will kill the boss

Technical Implementation

• Create boss sprite/graphics (5x scale)
• Add missile shooting behavior for boss
• Implement collision detection between:
  • Boss missiles and player
  • Boss and player
• Add game over trigger for both collision types