fix: auto-select firmware after upload, guard flash button without selection

- Auto-select uploaded firmware immediately after upload completes
- Auto-select single firmware on list load (no manual click needed)
- Hide flash button and show warning when no firmware is selected
- Re-apply manifest on list reload to avoid stale state
- Restore warning banner on firmware delete

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

frontend/js/app.js

@@ -288,6 +288,7 @@ uploadForm.addEventListener('submit', async e => {
     });
 
     await loadFirmwareList();
+    selectFirmware(data.id);   // 업로드 직후 자동 선택
     switchTab('tab-flash');
   } catch (err) {
     progressPct.textContent = `✗ ${err.message}`;
@@ -324,6 +325,8 @@ async function loadFirmwareList() {
     const resp = await fetch(`${API}/api/firmware`);
     state.firmwareList = await resp.json();
     renderFwList();
+    // 목록 로드 후 자동 선택 적용 (선택된 것이 있으면 manifest 재설정)
+    if (state.selectedFwId) selectFirmware(state.selectedFwId);
   } catch (err) {
     log(`✗ 펌웨어 목록 로드 실패: ${err.message}`, 'err');
   }
@@ -340,6 +343,11 @@ function renderFwList() {
     return;
   }
 
+  // 선택된 펌웨어가 없고 하나만 있으면 자동 선택
+  if (!state.selectedFwId && state.firmwareList.length === 1) {
+    state.selectedFwId = state.firmwareList[0].id;
+  }
+
   fwListEl.innerHTML = '';
   state.firmwareList.forEach(fw => {
     const el = document.createElement('div');
@@ -379,10 +387,14 @@ function selectFirmware(id) {
 
   const manifestUrl = `${location.origin}/api/firmware/${id}/manifest`;
   installBtn.setAttribute('manifest', manifestUrl);
-  $('#selected-fw-info').textContent = `선택됨: ${state.firmwareList.find(f=>f.id===id)?.name}`;
 
-  log(`✓ 플래시 대상 선택: ${state.firmwareList.find(f=>f.id===id)?.name}`, 'ok');
-  log(`  Manifest: ${manifestUrl}`, 'info');
+  const fw = state.firmwareList.find(f => f.id === id);
+  $('#selected-fw-info').textContent = `선택됨: ${fw?.name}`;
+  $('#flash-btn-wrap').style.display = 'block';
+  $('#no-fw-warning').style.display  = 'none';
+
+  log(`✓ 플래시 대상 선택: ${fw?.name}`, 'ok');
+  log(`  Manifest URL: ${manifestUrl}`, 'info');
 }
 
 async function deleteFirmware(id) {
@@ -392,7 +404,9 @@ async function deleteFirmware(id) {
     if (state.selectedFwId === id) {
       state.selectedFwId = null;
       installBtn.removeAttribute('manifest');
-      $('#selected-fw-info').textContent = '선택된 펌웨어 없음';
+      $('#selected-fw-info').textContent = '선택된 펌웨어 없음 — 위 목록에서 클릭하여 선택하세요';
+      $('#flash-btn-wrap').style.display  = 'none';
+      $('#no-fw-warning').style.display   = 'block';
     }
     await loadFirmwareList();
     log(`✓ 펌웨어 삭제 완료`, 'ok');