Fix default S3 region

[ruibaby]

Summary

• Change the default S3 region from Auto to us-east-1
• Fall back to us-east-1 when the Region field is left blank
• Update the Region field help text and README guidance

Rationale

Auto is not a valid SigV4 region for S3-compatible services that validate lowercase region names, such as RustFS. us-east-1 is a safer default and Cloudflare R2 aliases it to auto.

Testing

• ./gradlew test
• git diff --check

[longjuan]

/lgtm
这似乎是个break change，原来没填但能用的用户更新之后可能变成不能用了？

[longjuan]

我的又拍云之前使用的是region默认参数，用了这版插件后仍可正常上传。