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>

frontend/src/pages/admin/AccountsPage.tsx

@@ -88,7 +88,7 @@ export function UsersPage() {
   })
   const [inviteLoading, setInviteLoading] = useState(false)
   const [showCreateAccountModal, setShowCreateAccountModal] = useState(false)
-  const [createAccountForm, setCreateAccountForm] = useState({ name: '', plan: 'free' as 'free' | 'pro' | 'team' })
+  const [createAccountForm, setCreateAccountForm] = useState({ name: '', plan: 'free' as 'free' | 'pro' | 'team', owner_email: '' })
   const [createAccountLoading, setCreateAccountLoading] = useState(false)
 
   const fetchAccounts = useCallback(async () => {
@@ -223,13 +223,19 @@ export function UsersPage() {
       const created = await adminApi.createAccount({
         name: createAccountForm.name.trim(),
         plan: createAccountForm.plan,
+        owner_email: createAccountForm.owner_email.trim() || undefined,
       })
       toast.success('Account created')
       setShowCreateAccountModal(false)
-      setCreateAccountForm({ name: '', plan: 'free' })
+      setCreateAccountForm({ name: '', plan: 'free', owner_email: '' })
       navigate(`/admin/accounts/${created.id}`)
-    } catch {
-      toast.error('Failed to create account')
+    } catch (err: unknown) {
+      if (err && typeof err === 'object' && 'response' in err) {
+        const axiosErr = err as { response?: { data?: { detail?: string } } }
+        toast.error(axiosErr.response?.data?.detail || 'Failed to create account')
+      } else {
+        toast.error('Failed to create account')
+      }
     } finally {
       setCreateAccountLoading(false)
     }
@@ -634,6 +640,18 @@ export function UsersPage() {
               <option value="team">Team</option>
             </select>
           </div>
+          <div>
+            <label className="mb-1 block text-sm font-medium text-foreground">
+              Owner Email <span className="text-muted-foreground font-normal">(optional)</span>
+            </label>
+            <Input
+              type="email"
+              value={createAccountForm.owner_email}
+              onChange={(e) => setCreateAccountForm((form) => ({ ...form, owner_email: e.target.value }))}
+              placeholder="owner@example.com"
+            />
+            <p className="mt-1 text-xs text-muted-foreground">Must be an existing user.</p>
+          </div>
         </div>
       </Modal>
 

frontend/src/types/admin.ts

@@ -114,6 +114,7 @@ export interface AdminAccountDetailResponse extends AdminAccountListItem {
 export interface AdminAccountCreate {
   name: string
   plan: 'free' | 'pro' | 'team'
+  owner_email?: string
 }
 
 export interface AdminAccountUpdate {