Make GitW3 compatible with W3DS login

[plansombl]

Description

GitW3 (our Forgejo fork) should support W3DS login as an authentication method, allowing users to sign in using their W3DS identity rather than being limited to standard username/password or existing OAuth providers.

Reference

• Related issue: Make a fork of Forgejo and rename it to GitW3
• W3DS login integration: see ensombl/nextcloud-w3ds-login for prior art on how W3DS login has been integrated into other services.

Acceptance Criteria

• GitW3 accepts W3DS login as a valid authentication method
• Users can sign in to GitW3 using their W3DS identity
• W3DS login is available on the login page alongside any existing auth options
• Successful W3DS authentication creates/maps to a GitW3 user account
• Authentication flow is tested end-to-end in a staging environment

Desired Output (may vary)

A GitW3 instance where users can authenticate seamlessly via W3DS login, consistent with how W3DS login works across other MetaState/W3DS services.

🤖 Requested by @coodos via plansombl assistant