Hide register link when registration is disabled

[eleboucher]

Description

When a homeserver has registration disabled, the Login page still showed a "Do not have an account? Register" prompt that led to a dead-end Register page (which only renders "Registration has been disabled on this homeserver."). This hides that prompt when registration is genuinely unavailable, the server reports registration disabled (403 M_FORBIDDEN) and offers no SSO flow to register through. Password and SSO login are unaffected, and the Register page keeps its message as a fallback for direct navigation.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

AI disclosure:

• Partially AI assisted (clarify which code was AI assisted and briefly explain what it does).
• Fully AI generated (explain what all the generated code does in moderate detail).

code completion and 2nd review

[7w1]

lgtm, just add a changeset please