Organize composer into its own module

packages/app/src/composer/input/height-mirror.native.ts

@@ -0,0 +1,13 @@
+import type { RefObject } from "react";
+
+interface Args {
+  value: string;
+  textareaRef: RefObject<unknown>;
+  minHeight: number;
+  maxHeight: number;
+  onHeight: (height: number) => void;
+}
+
+export function useComposerHeightMirror(_args: Args): void {
+  // No-op on native: onContentSizeChange drives height natively.
+}