Add Score Multiplier Power-up

[dmccoystephenson]

Description

Implement a new power-up that temporarily multiplies the score gained.

Tasks

• Create new power-up type for score multiplication
• Implement score multiplier logic (2x for 10 seconds)
• Add visual effects for score multiplication
• Update scoring system to handle multipliers
• Add visual indicator for multiplied scores

Acceptance Criteria

• Score multiplier power-up appears with 15% spawn rate
• Points are doubled when multiplier is active
• Clear visual indication when multiplier is active
• Multiplier expires after 10 seconds
• Scores return to normal after multiplier expires