Fix duplicate Date header and add return path config

[daniel-lerch]

This pull request fixes a bug which caused all delivered emails to have a duplicate Date header. Furthermore, it is now possible to configure a separate address for Return-Path other than the From address.

[Copilot]

Pull request overview

This PR addresses outbound email correctness by preventing duplicate Date headers on forwarded messages and introducing a configurable SMTP envelope sender (Return-Path) separate from the From address.

Changes:

• Fixes Date header handling when creating forwarded MimeMessage instances to avoid duplicates.
• Adds EmailDelivery:ReturnPath configuration (plus documentation) and uses it for the SMTP envelope sender.
• Updates various dependency versions (webapp packages, NSwag, test SDK).

Reviewed changes

Copilot reviewed 8 out of 9 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
webapp/package.json	Bumps webapp version and updates several frontend dependencies.
webapp/package-lock.json	Lockfile updates reflecting the dependency upgrades.
server/Mailist/appsettings.json	Adds EmailDelivery.ReturnPath default setting.
server/Mailist/Mailist.csproj	Updates NSwag.AspNetCore package version.
server/Mailist/EmailRelay/MimeMessageCreationService.cs	Changes Date header assignment to avoid duplication.
server/Mailist/EmailDelivery/EmailDeliveryOptions.cs	Adds ReturnPath option to delivery configuration.
server/Mailist/EmailDelivery/EmailDeliveryJobController.cs	Uses ReturnPath (if set) as SMTP envelope sender.
server/Mailist.Tests/Mailist.Tests.csproj	Updates Microsoft.NET.Test.Sdk version.
docs/configuration.md	Documents EmailDelivery__ReturnPath.

Files not reviewed (1)

• webapp/package-lock.json: Language not supported

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