Participants and payments are now branch-scoped: migration adds nullable
branch_id FK to both tables, all creation paths (ParticipantForm,
receptionist wizards, InvoiceShow, POSService) pass branch_id through.

Error page copy button rewritten to use a hidden textarea rendered
server-side — eliminates JS string escaping issues with multi-line
error messages and special characters.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>