Attach dropped files in every composer (#1750)

* fix(app): attach dropped files in composer

Composer parents had to expose attachment callbacks, so New Workspace only wired image drops and JSON files never became file attachments. Centralize dropped-file ingestion in Composer so all composer surfaces share the same image and file path.

* fix(app): handle desktop dropped attachment paths

packages/app/src/attachments/picked-file.ts

@@ -0,0 +1,28 @@
+import { getFileExtension } from "@/attachments/file-types";
+import { copyDesktopAttachmentFile } from "@/desktop/attachments/desktop-file-commands";
+import { readDesktopFileBase64 } from "@/desktop/attachments/desktop-preview-url";
+
+export interface PickedFile {
+  fileName: string;
+  mimeType: string;
+  bytes: Uint8Array;
+}
+
+function base64ToUint8Array(base64: string): Uint8Array {
+  const binaryString = atob(base64);
+  const bytes = new Uint8Array(binaryString.length);
+  for (let i = 0; i < binaryString.length; i++) {
+    bytes[i] = binaryString.charCodeAt(i);
+  }
+  return bytes;
+}
+
+export async function readDesktopFileBytes(path: string): Promise<Uint8Array> {
+  const { path: managedPath } = await copyDesktopAttachmentFile({
+    attachmentId: crypto.randomUUID(),
+    sourcePath: path,
+    extension: getFileExtension(path) || null,
+  });
+  const base64 = await readDesktopFileBase64(managedPath);
+  return base64ToUint8Array(base64);
+}