chore(maic-editor): drop the now-dead addElement helper

`addElement` was only ever used by the inline image-insert which became
`insertImageElement`; text uses `armText` (toggle). PPTElement-typed
parameter was already unused after the text refactor — removing the dead
helper resolves the lint warning.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

components/edit/surfaces/slide/use-slide-surface.ts

@@ -29,8 +29,6 @@ export function buildInsertItems(
   // branch in useInsertFromCreateSelection adds the element at that rect.
   creatingType?: string,
 ): InsertPaletteItem[] {
-  const addElement = (element: PPTElement) =>
-    useSlideEditSession.getState().applyOp({ type: 'element.add', element });
   const armText = () => {
     const cs = useCanvasStore.getState();
     cs.setCreatingElement(creatingType === 'text' ? null : { type: 'text' });