Implement Dual-Panel Buying/Selling System

[mihaid11]

Current Limitations:

• Shop only supports buying items
• No player-to-shop selling functionality

Requested Changes:

• Dual-Panel Layout
  • Left Panel (Existing)
    • Maintains current buy functionality
    • Lists shopkeeper's items vertically
  • Right Panel (New)
    • Displays player's inventory
• Selling Mechanics
  • Hover Interaction
    • Displays item name, description, and sell price in a tooltip
  • Click Interaction
    • Sells one unit of the hovered item
    • Immediately adds sale price to the player's crystals
    • Removes sold item from inventory

Priority: Medium