fix: strip non-alphanumeric chars from admin username input

Logto usernames must match alphanumeric regex. The form now strips
invalid characters on input and shows a hint about the constraint.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/src/pages/vendor/CreateTenantPage.tsx

@@ -91,11 +91,11 @@ export function CreateTenantPage() {
             </select>
           </FormField>
 
-          <FormField label="Admin Username" htmlFor="admin-user" hint="Initial tenant admin (owner role)">
+          <FormField label="Admin Username" htmlFor="admin-user" hint="Initial tenant admin (owner role). Alphanumeric only, no hyphens.">
             <Input
               id="admin-user"
               value={adminUsername}
-              onChange={(e) => setAdminUsername(e.target.value)}
+              onChange={(e) => setAdminUsername(e.target.value.replace(/[^a-zA-Z0-9]/g, ''))}
               placeholder="admin"
             />
           </FormField>