feat(app): add drag-and-drop file upload and file picker to composer (#1474)

- Desktop web: drag-and-drop non-image files onto the chat pane to upload
- Desktop web: file picker option in the composer attachment sheet
- Uploaded files are sent to the LLM as filename references in a tmp directory
- New useFilePicker hook for cross-platform file picking
- Extended useFileDropZone to support generic file drops
- Added UploadedFileAttachment type to protocol and composer attachment system
- Fixed adaptive modal sheet closing on backdrop press in selection mode

packages/app/src/composer/actions.test.ts

@@ -179,6 +179,7 @@ function createFakeSendClient(
         throw options.rejection;
       }
     },
+    uploadFile: async () => ({ requestId: "test", file: null, error: null }),
   };
 }