fix: prevent editor blur on any click inside autocomplete menu

FocusTrap monitors focusin events and can redirect focus into the menu
container (blurring the editor) before individual MenuItem onMouseDown
handlers fire. Adding preventDefault at the container level ensures no
click anywhere inside the menu can steal focus from the editor.

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

src/app/components/editor/autocomplete/AutocompleteMenu.tsx

@@ -23,7 +23,7 @@ export function AutocompleteMenu({ headerContent, requestClose, children }: Auto
   };
 
   return (
-    <div className={css.AutocompleteMenuBase}>
+    <div className={css.AutocompleteMenuBase} onMouseDown={(e) => e.preventDefault()}>
       <div className={css.AutocompleteMenuContainer}>
         <FocusTrap
           focusTrapOptions={{