fix: add missing popoverFilter to handleKeyDown useCallback deps

Fixes stale closure bug where keyboard navigation in the slash command
popover could reference an outdated filteredItems list when the filter
text changed.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/components/chat/MessageInput.tsx

@@ -469,7 +469,7 @@ export function MessageInput({
         return;
       }
     },
-    [popoverMode, popoverItems, selectedIndex, insertItem, closePopover, badge, inputValue, removeBadge]
+    [popoverMode, popoverItems, popoverFilter, selectedIndex, insertItem, closePopover, badge, inputValue, removeBadge]
   );
 
   // Click outside to close popover