Added StarCoin system and test script

[sedemabla]

StarCoin currency system for Popstar Hub.

Features:

• Singleton with DontDestroyOnLoad (persists across scenes)
• AddCoins, SpendCoins, GetBalance, HasEnough
• PlayerPrefs persistence (coins save between sessions)
• Debug mode toggle from Inspector
• Events for UI integration:
  • OnCoinsEarned (for coin burst particles, floating "+100" text)
  • OnCoinsSpent (for spend animations, floating "-200" text)
  • OnPurchaseFailed (for error shake, flash red)
  • OnBalanceChanged (for keeping HUD updated)
  • OnCoinsReset (for reset animations)
• RewardFromScore for mini-game score conversion
• AddCoinsWithMultiplier for combo/streak bonuses
• GetDeficit for shop "you need X more" display
• CoinSystemTest script with keyboard controls (G/H/R/B)

How to use from any script:
StarCoinManager.Instance.AddCoins(100);
StarCoinManager.Instance.SpendCoins(200);
StarCoinManager.Instance.GetBalance();