fix(sheet): 머리 칸 / 로 층 나눔 · 줄 머리 칸 고정 · 줄 머리 칸 폭 자동 (M02 재검증)

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Tjoit7rxvpLMM7cafeVTo1

ui_template/sheet/ui_template_sheet.css

@@ -74,10 +74,10 @@
   text-align: center;
 }
 
-.ui-sheet__gutter {
+.ui-sheet__table .ui-sheet__gutter {
   position: sticky;
   left: 0;
-  z-index: 2;
+  z-index: 4;
   background: var(--color-surface) !important;
   color: var(--color-text-secondary);
   font-weight: 500;
@@ -176,6 +176,17 @@
   color: var(--color-accent);
 }
 
+.ui-sheet__headhint {
+  position: fixed;
+  z-index: 10;
+  padding: 2px 8px;
+  border: 1px solid var(--color-border);
+  border-radius: 4px;
+  background: var(--color-surface);
+  color: var(--color-text-secondary);
+  font-size: 0.76rem;
+}
+
 .ui-sheet__resize {
   position: absolute;
   top: 0;