feat: Level 13 - Spooky Night

[snackman]

Summary

• Adds shared special themed levels infrastructure (SpecialLevelConfig interface, SPECIAL_LEVELS registry, getSpecialLevel() helper) to constants.ts
• Implements Level 13 "Spooky Night": zombie horde with 60 customers, fast 800ms spawn interval, 0.6x speed multiplier, 75pt customer score override, green-tinted zombie sprites via hue-rotate filter, and a spooky purple background tint
• Threads level parameter through applyCustomerScoring, calculateCustomerScore, calculateLevelRewards to support per-level score/reward overrides
• Adds missedPenaltyStars support to STAR_LOST handlers for future special levels that penalize harder
• Updates GameBoard to render background with CSS filter + tint overlay for special levels, and show special level name/subtitle in the level announcement

This is the first of 3 PRs split from PR #31 (special themed levels). This PR contains all shared infrastructure plus the Level 13 config. Levels 25 and 50 will follow in separate PRs.

Test plan

• Verify build succeeds (vite build passes)
• Confirm levels 1-12 play identically to main (no regressions)
• Reach level 13 and confirm: spooky purple tint, zombie sprites with green hue, fast spawn, "Spooky Night" announcement
• Verify level 13 customers are all normal/zombie (no special variants)
• Verify level 14+ returns to normal gameplay

🤖 Generated with Claude Code

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Project	Deployment	Actions	Updated (UTC)
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