refactor(ai-toolbar): stack attachment previews by row

src/components/ai-toolbar.tsx

@@ -466,9 +466,7 @@ export const AIToolbar = ({
 									animate={{ opacity: 1, y: 0 }}
 									exit={{ opacity: 0, y: 4 }}
 									transition={{ duration: 0.15 }}
-									className={
-										isMobile ? "mt-3 grid gap-2" : "mt-3 grid gap-2 grid-cols-2"
-									}
+									className="mt-3 grid gap-2"
 								>
 									{imagePreviews.map((preview, index) => (
 										<motion.div