feat: category scopes, group-admin role, and permission model

- Three category scopes: personal / group / system (watch)
- PascalCase-with-dashes naming convention enforced at backend + frontend
- is_group_admin flag on GroupMembership; PATCH endpoint for admins to toggle it
- Categories router: scope-based list/create/rename/delete with _check_can_manage_cat
- Documents router: delete uses is_admin + can_delete share flag + group-admin check; remove_category requires doc ownership; assign_category accepts group/system categories
- Proxy layers inject x-user-is-admin and x-user-admin-groups headers
- Frontend: ManageCategoriesDialog grouped by scope with lock icons; SourcePanel scope picker + client-side name validation; AdminGroupsPage group-admin checkbox

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/schemas/user.py

@@ -122,6 +122,7 @@ class UserGroupOut(BaseModel):
     id: str
     name: str
     description: str | None
+    is_group_admin: bool = False
 
     model_config = {"from_attributes": True}