[conducted] Create Rail Mart shop interface and economy

[githumps]

Summary

• implement a buy/sell menu UI accessible from Rail Mart clerks that supports desktop and mobile controls
• hook into the player's inventory/money to add and remove items, enforcing stock/price data from a config table
• provide feedback messages for successful transactions, insufficient funds, and full inventory states

Why

• items like Trainballs and Potions are key to progression; currently there is no legitimate way to obtain them mid-game
• the economy loop also encourages exploration and trainer battles for money rewards

Acceptance Criteria

• entering a Rail Mart and interacting with the clerk opens the shop UI with categories (at minimum, Trainballs and healing items)
• purchases deduct money and add items; sales credit money and remove inventory
• menu navigation works with keyboard and on-screen mobile controls

Notes

• UI hooks likely live in js/ui.js with inventory logic in js/player.js
• relates to [conducted] Expand NPC Interactions and Dialogue System #2 and [conducted] Enhance UI/Menu System #7