Convert from class-based to object-based export for consistency with
other services (economy, inventory, quest, etc).
Changes:
- Move sessions Map and helper functions to module scope
- Convert static methods to object properties
- Update executeTrade to use withTransaction helper
- Update all imports from TradeService to tradeService
Updated files:
- trade.service.ts (main refactor)
- trade.interaction.ts (update usages)
- trade.ts command (update import and usage)
All tests passing with no breaking changes.
