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>