fix: standardize error classes in shared service modules

Replace raw `Error` with `UserError` for user-facing conditions (invalid trade state, user not found, permission/channel type checks) and `SystemError` for internal failures (DB insert failures, external API errors, missing config). Improves Discord UX by ensuring user-facing errors are surfaced cleanly via withCommandErrorHandling.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

shared/modules/economy/economy.service.ts

@@ -96,7 +96,7 @@ export const economyService = {
             });
 
             if (!user) {
-                throw new Error("User not found");
+                throw new UserError("User not found");
             }
 
             let streak = (user.dailyStreak || 0) + 1;