fix: Ctrl+S in SECTION-TAB stale closure, saveSectionTab uses initial sessionTabs

Keyboard handler (useEffect [] deps) captures handleSaveSectionTab
from first render, which has sessionTabs = []. When section tab
is opened later, saved handleSaveSectionTab finds no tab and
returns silently. Added handleSaveSectionTabRef, updated on
every render, used in keyboard handler via ref.current.

app/static/js/app.jsx

@@ -8068,7 +8068,7 @@ const App = () => {
         e.preventDefault();
         var curTab = activeTabRef.current;
         if (curTab.startsWith('session_')) {
-          handleSaveSectionTab(curTab);
+          handleSaveSectionTabRef.current(curTab);
           showToast('Đã lưu Section', 'success');
         } else {
           handleExportSFS();
@@ -8177,7 +8177,7 @@ const App = () => {
           });
         } else if (curTab.startsWith('session_')) {
           // Section tab: save section
-          handleSaveSectionTab(curTab);
+          handleSaveSectionTabRef.current(curTab);
           showToast('Đã lưu Section', 'success');
         } else {
           // Sub-tab: save current tab
@@ -11700,6 +11700,8 @@ const App = () => {
   };
 
   // ── Add item to selectedItemIds ──
+  const handleSaveSectionTabRef = useRef(handleSaveSectionTab);
+  handleSaveSectionTabRef.current = handleSaveSectionTab;
   const handleAddToSelection = itemId => {
     setSelectedItemIds(prev => {
       var next = new Set(prev);