refactor: convert TradeService to object export pattern

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.

src/commands/economy/trade.ts

@@ -1,6 +1,6 @@
 import { createCommand } from "@/lib/utils";
 import { SlashCommandBuilder, ChannelType, ThreadAutoArchiveDuration, MessageFlags } from "discord.js";
-import { TradeService } from "@/modules/trade/trade.service";
+import { tradeService } from "@/modules/trade/trade.service";
 import { getTradeDashboard } from "@/modules/trade/trade.view";
 import { createErrorEmbed, createWarningEmbed } from "@lib/embeds";
 
@@ -59,7 +59,7 @@ export const trade = createCommand({
         }
 
         // Setup Session
-        const session = TradeService.createSession(thread.id,
+        const session = tradeService.createSession(thread.id,
             { id: interaction.user.id, username: interaction.user.username },
             { id: targetUser.id, username: targetUser.username }
         );