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/components/message.tsx

@@ -443,6 +443,8 @@ function getUserMessageAttachmentLabel(
       return `Issue #${attachment.number}`;
     case "text":
       return attachment.title ?? t("message.attachments.textAttachment");
+    case "uploaded_file":
+      return attachment.fileName;
     default:
       return "";
   }