feat: Let modules inject admin tabs when editing a user group

[hellopablo]

Summary

• Adds Nails\Auth\Interfaces\Admin\User\Group\Tab, mirroring user edit tabs so modules can contribute group settings.
• Discovers those tabs when editing a group, runs their validation, and calls afterSave() after the group is written.
• Replaces the hard-coded legacy 2FA tab with the injected tabs.

Test plan

• Edit a user group and confirm Basic / Password / Permissions still work
• With module-multi-factor-auth installed, confirm the MFA tab appears and saving a policy persists

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