Function transferDownload

transferDownload(
    profileId: string,
    bucket: string,
    key: string,
    destPath: string,
): Promise<string>
Initiate a streaming download of key from bucket to destPath.

Returns the requestId (UUID v4) immediately. The actual download runs in a background Rust task. Listen for progress and state updates via:
```
const unlisten = await listen<TransferProgressPayload>(
  "transfer:progress",
  (event) => {
    if (event.payload.requestId === requestId) { ... }
  }
);
```
Parameters
- profileId: string
  The profile whose credentials to use.
- bucket: string
  The bucket name.
- key: string
  The S3 object key to download.
- destPath: string
  Local filesystem path where the file should be written.
Returns Promise<string>
The requestId to correlate progress/state events.
- Defined in src/api/transfers.ts:166