feat: QR/URL login sharing

[PartyDonut]

Pull Request Description

Add a feature where the server admin can share login-links with other users for easier logging-in.
Users can share the URL itself or a QR-code that they can use to login.

• If a new package was added, did you ensure it works for all supported platforms? Is the package well maintained
• Check that any changes are related to the issue at hand.

[github-actions]

Dart Analyzer found 1 issue

• 0 error.
• 1 warning.
• 0 info log.
• 0 formatting issue.

---

• Warning - lib/screens/control_panel/widgets/auth_link_dialog.dart:94:12 - 'PrettyQrShape.custom' is experimental and could be removed or changed at any time. (experimental_member_use). See link or link.