feat(travel): 사진 그리드 안정화 + 앨범 커버 지정 버튼 + 동기화 결과 개선

- MasonryGrid: CSS columns → CSS Grid로 전환 (스크롤 시 정렬 위치 변동 방지)
- HeroLightbox: "커버로 지정" 버튼 추가 (PUT /api/travel/albums/{album}/cover 호출)
- Travel: 동기화 토스트에 신규 폴더 발견 수 표시

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

src/pages/travel/HeroLightbox.css

@@ -59,6 +59,41 @@
   font-weight: 600;
 }
 
+/* ── Cover button ── */
+.hero-lb__cover-btn {
+  padding: 6px 14px;
+  border-radius: 8px;
+  border: 1px solid rgba(255, 255, 255, 0.15);
+  background: rgba(255, 255, 255, 0.08);
+  color: var(--tv-muted, rgba(232, 221, 208, 0.45));
+  font-family: var(--tv-mono, 'Space Mono', 'Courier New', monospace);
+  font-size: 11px;
+  letter-spacing: 0.04em;
+  cursor: pointer;
+  transition: background 0.2s, color 0.2s, border-color 0.2s;
+  white-space: nowrap;
+  flex-shrink: 0;
+}
+.hero-lb__cover-btn:hover:not(:disabled) {
+  background: rgba(255, 255, 255, 0.16);
+  color: var(--tv-text, #e8ddd0);
+  border-color: rgba(255, 255, 255, 0.3);
+}
+.hero-lb__cover-btn:disabled {
+  opacity: 0.5;
+  cursor: not-allowed;
+}
+.hero-lb__cover-btn--done {
+  border-color: rgba(200, 144, 94, 0.5);
+  color: #c8905e;
+  background: rgba(200, 144, 94, 0.12);
+}
+.hero-lb__cover-btn--error {
+  border-color: rgba(220, 80, 80, 0.5);
+  color: #dc5050;
+  background: rgba(220, 80, 80, 0.12);
+}
+
 .hero-lb__close {
   width: 40px;
   height: 40px;

src/pages/travel/HeroLightbox.jsx

@@ -51,6 +51,8 @@ export default function HeroLightbox({
   const pendingAdvanceRef = useRef(false);
   const stripRef = useRef(null);
   const prevOverflowRef = useRef('');
+  const [coverStatus, setCoverStatus] = useState(null); // 'saving' | 'done' | 'error'
+  const coverTimerRef = useRef(null);
   const accent = useMemo(() => getRegionAccent(regionId), [regionId]);
   const reduced = useMemo(() => prefersReduced(), []);
   const animMs = reduced ? 0 : ANIM_MS;
@@ -146,6 +148,28 @@ export default function HeroLightbox({
     delta: 30,
   });
 
+  /* — Set as album cover — */
+  const handleSetCover = useCallback(async () => {
+    const p = photos[selectedIndex];
+    if (!p || !albumName || coverStatus === 'saving') return;
+    const filename = p.file || p.filename || p.name || '';
+    if (!filename) return;
+    setCoverStatus('saving');
+    try {
+      const res = await fetch(`/api/travel/albums/${encodeURIComponent(albumName)}/cover`, {
+        method: 'PUT',
+        headers: { 'Content-Type': 'application/json' },
+        body: JSON.stringify({ filename }),
+      });
+      if (!res.ok) throw new Error(`${res.status}`);
+      setCoverStatus('done');
+    } catch {
+      setCoverStatus('error');
+    }
+    if (coverTimerRef.current) clearTimeout(coverTimerRef.current);
+    coverTimerRef.current = setTimeout(() => setCoverStatus(null), 2000);
+  }, [selectedIndex, photos, albumName, coverStatus]);
+
   /* — Current photo — */
   const photo = photos[selectedIndex];
   if (!photo) return null;
@@ -183,6 +207,17 @@ export default function HeroLightbox({
             {' / '}
             {photos.length}
           </span>
+          <button
+            className={`hero-lb__cover-btn${coverStatus === 'done' ? ' hero-lb__cover-btn--done' : coverStatus === 'error' ? ' hero-lb__cover-btn--error' : ''}`}
+            onClick={handleSetCover}
+            disabled={coverStatus === 'saving'}
+            title="이 사진을 앨범 커버로 지정"
+          >
+            {coverStatus === 'saving' ? '저장 중…'
+              : coverStatus === 'done' ? '커버 지정됨'
+              : coverStatus === 'error' ? '실패'
+              : '커버로 지정'}
+          </button>
           <button
             className="hero-lb__close"
             onClick={handleClose}

src/pages/travel/MasonryGrid.css

@@ -1,18 +1,18 @@
-/* ── MasonryGrid ── */
+/* ── MasonryGrid — stable CSS Grid layout ── */
 
 .masonry-grid {
-  column-count: 4;
+  display: grid;
-  column-gap: 8px;
+  grid-template-columns: repeat(4, 1fr);
+  gap: 8px;
 }
 
 /* item */
 .masonry-item {
-  break-inside: avoid;
-  margin-bottom: 8px;
   position: relative;
   border-radius: 4px;
   overflow: hidden;
   cursor: zoom-in;
+  aspect-ratio: 4 / 3;
 
   /* scroll-reveal initial state */
   opacity: 0;
@@ -28,7 +28,8 @@
 .masonry-item__img {
   display: block;
   width: 100%;
-  height: auto;
+  height: 100%;
+  object-fit: cover;
   transition: filter 0.25s ease;
 }
 
@@ -62,12 +63,12 @@
 /* sentinel */
 .masonry-sentinel {
   height: 1px;
-  column-span: all;
+  grid-column: 1 / -1;
 }
 
 /* loading dots */
 .masonry-loading {
-  column-span: all;
+  grid-column: 1 / -1;
   display: flex;
   justify-content: center;
   gap: 6px;
@@ -97,7 +98,7 @@
 
 /* end message */
 .masonry-end {
-  column-span: all;
+  grid-column: 1 / -1;
   text-align: center;
   font: 11px var(--tv-mono);
   letter-spacing: 0.12em;
@@ -109,13 +110,13 @@
 /* responsive */
 @media (max-width: 1024px) {
   .masonry-grid {
-    column-count: 3;
+    grid-template-columns: repeat(3, 1fr);
   }
 }
 
 @media (max-width: 768px) {
   .masonry-grid {
-    column-count: 2;
+    grid-template-columns: repeat(2, 1fr);
   }
 }
 

src/pages/travel/Travel.jsx

@@ -133,7 +133,7 @@ const Travel = () => {
                         <div className={`tv-sync-toast${syncResult.error ? ' is-error' : ''}`}>
                             {syncResult.error
                                 ? `동기화 실패: ${syncResult.error}`
-                                : `+${syncResult.added} 추가 / ${syncResult.removed} 삭제 / 썸네일 ${syncResult.thumbs_generated}개 (${syncResult.duration_sec}s)`
+                                : `+${syncResult.added} 추가 / ${syncResult.removed} 삭제 / 썸네일 ${syncResult.thumbs_generated}개${syncResult.discovered ? ` / 신규 폴더 ${syncResult.discovered}개 발견` : ''} (${syncResult.duration_sec}s)`
                             }
                         </div>
                     )}