Add Invincibility Power-up

[dmccoystephenson]

Description

Create a power-up that temporarily makes the snake invincible to collisions.

Tasks

• Implement invincibility power-up
• Add visual effects for invincible state
• Modify collision detection to handle invincibility
• Add spawn rate of 5%
• Create duration timer (3 seconds)

Acceptance Criteria

• Invincibility power-up spawns correctly
• Snake doesn't die from collisions while invincible
• Clear visual indication of invincibility state
• Invincibility expires after 3 seconds
• Proper return to normal state after expiration