Customize mode hardcodes SigV4 region to us-east-1, breaks all non-AWS S3 providers (Scaleway, etc.)

[tijs]

When the Customize toggle is enabled in the Amazon S3 host config, uPic forces the SigV4 signing region to us-east-1 regardless of the configured endpoint. Many S3-compatible providers reject signatures whose region doesn't match the bucket's region, which makes Customize mode unusable for them.

Reproduction (Scaleway Object Storage)

1. Create a bucket in region pl-waw (Warsaw).
2. uPic → Preferences → Host → + Amazon S3 → check Customize.
3. Endpoint: s3.pl-waw.scw.cloud, Bucket: tijs-upic, valid Access/Secret keys, Domain: https://tijs-upic.s3.pl-waw.scw.cloud.
4. Click Validate.

Expected: validation succeeds.
Actual: error 26. Server returns 403 AccessDenied because the SigV4 signature is signed with us-east-1 instead of pl-waw.

Verification that credentials and policy are correct

The same Access/Secret keys, signed manually with region=pl-waw, succeed:

PUT /tijs-upic/uPic/test.txt → 200 OK

Signed with region=us-east-1, the same request fails:

PUT /tijs-upic/uPic/test.txt → 403 AccessDenied

Only the signing region is wrong.

Root cause

uPic/Models/S3/S3Uploader.swift ~line 39:

var region = config.region == nil ? .useast1 : SotoS3.Region(rawValue: config.region!)
var endpoint = config.endpoint

if customize {
    region = .useast1     // <-- forced
} else {
    endpoint = S3Region.endPoint(config.region)
}

And S3ConfigView.swift::changeCustomizeModel clears region to nil when Customize is toggled on, so the user has no surface to set a non-AWS region.

Proposed fix

Either:

1. Add a Region text field to the Customize panel (visible only when Customize is on), and remove the region = .useast1 override in S3Uploader.swift. Users provide both endpoint and region — matches aws-cli, rclone, every other S3 client.
2. Auto-derive the region from the endpoint hostname as a fallback.

Option 1 is more explicit. Happy to open a PR.

Affected providers

• Scaleway Object Storage (pl-waw, fr-par, nl-ams)
• MinIO with region enforcement
• Wasabi in some regions
• Any S3-compatible provider that validates signing region

Environment

• uPic 0.21.1 (Homebrew)
• macOS 15.x (Darwin 25.3.0)