Participants are created with externalId=null by default, causing the
LLWS simulator to crash at runtime. Fix in two parts:
1. Name-prefix inference: if externalId is null, participants whose
name starts with "US " or equals "US" are assigned to the US side;
all others are assigned to Intl. Pools are always randomized when
inferred (no pool suffix).
2. Admin UI: add an inline-editable "Group" column to the Manage
Participants page so admins can explicitly set externalId for any
simulator that uses it (LLWS, and future cases like NHL conferences).
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>
68b2e070e6