fix(app): stop composer height flicker

Native scrollbar appearance changed the textarea's usable width, feeding alternating measurements back into the composer height mirror. Reserve the gutter so wrapping and height remain stable.

packages/app/src/styles/install-web-scrollbar-styles.web.ts

@@ -19,6 +19,10 @@ export function installWebScrollbarStyles(): () => void {
   scrollbar-width: ${WEB_SCROLLBAR_WIDTH};
 }
 
+[data-composer-input] {
+  scrollbar-gutter: stable;
+}
+
 *::-webkit-scrollbar {
   width: ${WEB_SCROLLBAR_SIZE_PX}px;
   height: ${WEB_SCROLLBAR_SIZE_PX}px;