AWS S3 upload example

Demonstrates the @uppy/aws-s3 plugin with two signing modes.

Sign on the client (STS credentials)

Uses getCredentials to fetch temporary STS credentials from /s3/sts. The browser signs all S3 requests locally using SigV4.

Sign on the server (presigned URLs)

Uses signRequest to call /s3/presign for each S3 operation. The server generates presigned URLs; the browser uses them directly.