fix: display platform registration errors inside the creation modal

src/pages/platforms/PlatformsPage.tsx

@@ -68,6 +68,7 @@ export function PlatformsPage({ session }: { session: AuthSession }) {
   const [selectedMasterId, setSelectedMasterId] = useState('');
   const [selectedAgentId, setSelectedAgentId] = useState('');
   const [platformName, setPlatformName] = useState('');
+  const [createError, setCreateError] = useState<string | null>(null);
   const [agentsLoading, setAgentsLoading] = useState(false);
   const [agentsError, setAgentsError] = useState<string | null>(null);
   const [registrationCode, setRegistrationCode] = useState<string | null>(null);
@@ -203,6 +204,7 @@ export function PlatformsPage({ session }: { session: AuthSession }) {
   };
 
   const handleOpenCreateModal = () => {
+    setCreateError(null);
     setModalOpen(true);
     setSelectedAgentId('');
     setAgents([]);
@@ -222,6 +224,7 @@ export function PlatformsPage({ session }: { session: AuthSession }) {
   };
 
   const handleCreatePlatform = async () => {
+    setCreateError(null);
     if (!selectedMasterId) return;
     if (!selectedAgentId) {
       setAgentsError('플랫폼에 연결할 에이전트를 선택해 주세요.');
@@ -244,7 +247,7 @@ export function PlatformsPage({ session }: { session: AuthSession }) {
         message: created?.name ?? masterPlatforms.find((platform) => platform.id === selectedMasterId)?.name,
       });
     } catch (reason) {
-      setError(reason instanceof Error ? reason.message : '플랫폼을 등록하지 못했습니다.');
+      setCreateError(reason instanceof Error ? reason.message : '플랫폼을 등록하지 못했습니다.');
     } finally {
       setSaving(false);
     }
@@ -674,6 +677,11 @@ export function PlatformsPage({ session }: { session: AuthSession }) {
                   placeholder="예: F1Tenth #01"
                 />
               </label>
+              {createError ? (
+                <p role="alert" className="mt-3 rounded-md border border-[#fecdd3] bg-[#fff1f2] px-3 py-2 text-[12px] font-bold text-[#be123c]">
+                  {createError}
+                </p>
+              ) : null}
             </div>
 
             <div className="flex justify-end gap-2 border-t border-[#cbd5e1] px-5 py-4">