fix: sub-tab keyboard block captures Ctrl+A/S in SECTION-TAB

Line 7942 checked activeTabRef.current !== 'main', which
is true for SECTION-TAB (session_xxx). Sub-tab block
only handles audio/image keys then unconditionally
returns at line 8005 -> Ctrl+A/S never reached.
Changed to exclude section tabs via
!activeTabRef.current.startsWith('session_').

app/static/js/app.jsx

@@ -7939,7 +7939,7 @@ const App = () => {
         if (handlePlayPauseRef.current) handlePlayPauseRef.current();
         return;
       }
-      if (activeTabRef.current !== 'main') {
+      if (activeTabRef.current !== 'main' && !activeTabRef.current.startsWith('session_')) {
         // Sub-Tab keyboard shortcuts mapping
         const curTabId = activeTabRef.current;
         if (ctrl && alt && e.key === 'n') {