Adds IsPrimary to OrganizationUser (per-user flag, one primary per user).
First org created is automatically set as primary. Selecting a different
org via the UI calls PUT /organisations/{id}/primary to persist the choice
server-side. Dashboard auto-selects: localStorage org (refreshed) →
primary org → single org when no explicit selection exists.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
