feat(admin): allow setting owner when creating an account

Adds optional owner_email field to the Create Account modal. Superadmin
can specify an existing user's email to assign as account owner at
creation time. Backend 404s with a clear message if the email is unknown.
Error detail now surfaces to the toast instead of a generic message.

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

backend/app/api/endpoints/admin.py

@@ -431,10 +431,19 @@ async def create_account(
     current_user: Annotated[User, Depends(require_admin)],
 ):
     """Create a new account without requiring an initial user."""
+    owner_id = None
+    if data.owner_email:
+        result = await db.execute(select(User).where(User.email == data.owner_email.strip()))
+        owner = result.scalar_one_or_none()
+        if not owner:
+            raise HTTPException(status_code=status.HTTP_404_NOT_FOUND, detail=f"No user found with email '{data.owner_email}'")
+        owner_id = owner.id
+
     display_code = await _generate_unique_display_code(db)
     new_account = Account(
         name=data.name.strip(),
         display_code=display_code,
+        owner_id=owner_id,
     )
     db.add(new_account)
     await db.flush()
@@ -448,7 +457,7 @@ async def create_account(
 
     await log_audit(
         db, current_user.id, "account.create_admin", "account", new_account.id,
-        {"name": new_account.name, "plan": data.plan},
+        {"name": new_account.name, "plan": data.plan, "owner_email": data.owner_email},
     )
     await db.commit()
     return await _get_account_detail_payload(new_account.id, db)