ProfileOrganizerItem.display_name crashes when null (backend sends null)

## Bug Frontend types `display_name` as required `string` but backend can return `null` (when user has not set a display name). ## Impact - Renders literal "null" text in organizer cards - May crash on `.trim()`, `.length`, or other string methods ## Fix Change frontend type from `display_name: string` to `display_name: string | null` and add a fallback (e.g., show username or "Unknown"). ## Location - Frontend: `features/organizations/api/organizations.profile.api.ts` → `ProfileOrganizerItem` - Backend: `app/schemas/organization.py` → `ProfileOrganizerItem.display_name: str | None` ## Found by API contract audit (2026-03-19)