fix(app): paste clipboard images on mobile (#2793)

Native composers had no path from the device clipboard into image attachments. Route clipboard data URLs through the existing attachment persistence flow and expose the action only in the native attachment sheet.

packages/app/src/composer/clipboard-image.ts

@@ -0,0 +1,29 @@
+import type { PickedImageAttachmentInput } from "@/hooks/image-attachment-picker";
+
+interface ClipboardImage {
+  data: string;
+}
+
+export interface ClipboardImageReader {
+  hasImageAsync: () => Promise<boolean>;
+  getImageAsync: (options: { format: "png" }) => Promise<ClipboardImage | null>;
+}
+
+export async function readClipboardImage(
+  clipboard: ClipboardImageReader,
+): Promise<PickedImageAttachmentInput | null> {
+  if (!(await clipboard.hasImageAsync())) {
+    return null;
+  }
+
+  const image = await clipboard.getImageAsync({ format: "png" });
+  if (!image) {
+    return null;
+  }
+
+  return {
+    source: { kind: "data_url", dataUrl: image.data },
+    mimeType: "image/png",
+    fileName: "clipboard.png",
+  };
+}