Add money/currency system with battle rewards and spending

[githumps]

Summary

Implement the money system where players earn cash from trainer battles and spend it at Rail Marts for items.

Why

Economic progression is important for item access. Players need money to buy Trainballs, Potions, and other supplies.

Acceptance Criteria

• Player has money counter that persists in save data
• Defeating trainers awards prize money based on trainer level
• Money displays in menu and during shop transactions
• Can spend money at shops, cannot buy if insufficient funds
• Money cannot go negative
• Display money earned at end of trainer battles

Notes

• Integrates with trainer battles and shop system ([conducted] Create Rail Mart shop interface and economy #24)
• Prize money formula: Trainer level * Base payout * trainer tier
• Related to save data expansion ([conducted] Extend save data schema for badges, story flags, and PC storage #38)