fix: add missing 'S' keyboard shortcut for split at cursor

Added keyboard handler for the 'S' key to trigger split at cursor.
The shortcut was defined in the command palette but missing from
the keyboard event handler.

Note: Ctrl+A for Select All was already working correctly.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

components/editor/AudioEditor.tsx

@@ -1844,6 +1844,13 @@ export function AudioEditor() {
         return;
       }
 
+      // S: Split at cursor
+      if (e.key === 's' && !e.ctrlKey && !e.metaKey) {
+        e.preventDefault();
+        handleSplitAtCursor();
+        return;
+      }
+
       // ?: Open keyboard shortcuts help
       if (e.key === '?' && !e.ctrlKey && !e.metaKey) {
         e.preventDefault();