feat: add Ctrl+A select all items and Del key delete selected items/groups

- Ctrl+A already existed for selecting all timeline items across tracks
- Added Del/Backspace key handler for selectedItemIds: deletes all selected sections, MIDI items, and audio clips from the selected track(s)
- Clears selection after deletion and shows toast with count

app/static/js/app.jsx

@@ -8158,6 +8158,40 @@ const App = () => {
         return;
       }
        if (e.key === 'Delete' || e.key === 'Backspace' || e.key === 'Del') {
+         if (selectedItemIds && selectedItemIds.size > 0) {
+           e.preventDefault();
+           const idsToDelete = new Set(selectedItemIds);
+           setSelectedItemIds(new Set());
+           updateActiveTracks(prev => prev.map(t => {
+             let changed = false;
+             const sections = (t.sections || []).filter(s => {
+               if (idsToDelete.has(s.id)) { changed = true; return false; }
+               return true;
+             });
+             const midiItems = (t.midiItems || []).filter(m => {
+               if (idsToDelete.has(m.id)) { changed = true; return false; }
+               return true;
+             });
+             const clips = (t.clips || []).filter(c => {
+               const canonical = c.id === 'default' ? 'default_' + t.id : c.id;
+               if (idsToDelete.has(canonical)) { changed = true; return false; }
+               return true;
+             });
+             if (!changed) return t;
+             return {
+               ...t,
+               sections,
+               midiItems,
+               clips,
+               buffer: clips[0]?.buffer || null,
+               startTime: clips[0]?.startTime || 0,
+               name: clips[0]?.name || t.name
+             };
+           }));
+           const count = idsToDelete.size;
+           showToast(count === 1 ? 'Đã xóa 1 item.' : `Đã xóa ${count} items.`, 'info');
+           return;
+         }
          const selClip = selectedClipIdRef.current;
         if (selClip) {
           e.preventDefault();