feat(users): support creating app users with a Firebase Auth account

New users can now be created with a password, which creates a real
Firebase Auth account (so they can log into the mobile app immediately)
alongside the Firestore profile document. UserCreate is now
UserProfile + password (request-only, never persisted or echoed back);
deleting a user also removes their Auth account.

- backend/users: split UserCreate into UserProfile (persisted shape)
  and UserCreate (adds password), wire firebase_auth create/delete
- CreateUserModal: new lightweight modal for creating a user from
  other flows (e.g. device onboarding) without leaving the page
- UserForm: adds the password field for new users; also fixes
  useToast() being used undestructured (toast.success(...) was being
  called on the hook's return value instead of its .toast method)

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/pages/bellcloud/users/UserForm.jsx

@@ -71,7 +71,7 @@ function StatusToggle({ value, onChange }) {
 
 function PhotoField({ value, onChange, userId }) {
   const fileInputRef              = useRef(null)
-  const toast                     = useToast()
+  const { toast }                 = useToast()
   const [uploading, setUploading] = useState(false)
   const [hovered,   setHovered]   = useState(false)
 
@@ -192,7 +192,7 @@ function EyeToggle({ show, onToggle }) {
 }
 
 function PasswordCard({ userId, hasUid }) {
-  const toast                       = useToast()
+  const { toast }                   = useToast()
   const [password,  setPassword]    = useState('')
   const [confirm,   setConfirm]     = useState('')
   const [showPass,  setShowPass]    = useState(false)
@@ -302,9 +302,9 @@ function PasswordCard({ userId, hasUid }) {
 // ─── Main ────────────────────────────────────────────────────────────────────
 
 export default function UserForm() {
-  const { id }   = useParams()
-  const navigate = useNavigate()
-  const toast    = useToast()
+  const { id }    = useParams()
+  const navigate  = useNavigate()
+  const { toast } = useToast()
   const isEdit   = Boolean(id)
 
   const [form, setForm] = useState({
@@ -317,6 +317,7 @@ export default function UserForm() {
     userTitle:        '',
     settingsPIN:      '',
     quickSettingsPIN: '',
+    password:         '',
   })
   const [uid,     setUid]     = useState('')
   const [loading, setLoading] = useState(false)
@@ -435,6 +436,18 @@ export default function UserForm() {
                   placeholder="+1 234 567 8900"
                 />
               </div>
+              {!isEdit && (
+                <FormField
+                  label="Password"
+                  name="password"
+                  type="password"
+                  value={form.password}
+                  onChange={set('password')}
+                  required
+                  placeholder="Min. 6 characters"
+                  hint="Used to sign into the mobile app immediately — this creates the Firebase Auth account."
+                />
+              )}
               <StatusToggle value={form.status} onChange={setVal('status')} />
             </div>
           </Card>