[conducted] Implement Rail Mart shop system for buying items

[githumps]

Summary

Implement shop NPC where players can spend money to buy Potions, Super Potions, and Boxcars.

Requirements

• Shop NPC in Rail Mart building (Piston Town)
• Menu UI showing items for sale with prices
• Purchase flow: Select item → Confirm → Deduct money → Add to inventory
• Validation: Cannot buy if insufficient funds
• Validation: Cannot buy if inventory would exceed max (optional cap)

Proposed Prices

• Potion: $300 (heals 20 HP)
• Super Potion: $700 (heals 50 HP)
• Boxcar: $200 (catches wild trains)

Implementation

• js/game.js: Add SHOP state
• js/ui.js: Add drawShop() method
• js/world-maps.js: Add Rail Mart interior with NPC
• NPC interaction triggers SHOP state

Scope

~120 lines total (game.js + ui.js)

Depends On

• Add money/currency system with battle rewards and spending #53 (money system must track balance)
• Completed: Bag UI (Add pause menu with Trains/Bag/Save/Options #57)

Testing

• Enter shop
• Navigate items with arrow keys
• Attempt purchase with sufficient funds → success
• Attempt purchase with insufficient funds → blocked
• Verify inventory updated
• Verify money deducted