fix(invites): resolve the runtime configuration form on self hosted instances.

[notrevenant]

Note

Closes #1217

Summary

• What changed:
  Modified InviteRegisterPage.tsx to respect the instance's registration settings. When the instance is set to "invite-only" the page now shows the proper registration form instead of allowing open registration (display name).

• Why it is correct:
  Previously, the invite flow ignored the RuntimeConfig settings and always allowed registration. It only collected a display name, which wasn't sufficient for account creation if the instance was set to invite only. Now it uses the same registration flow as the index page.

• Risk:
  Low as it reuses existing, tested registration logic from other code (AuthMinimalRegisterFormore.tsx).

• What I didn't do:

1. I didn't fully hide the registration if the instance is closed as this doesn't seem to be the case on the homepage either, so I let it be.
2. I didn't modify the register form's design.

Verification

• Tests run:
  None
• Manual checks:
  Used docker to custom build the frontend and ran a test on a "incognito" tab to clear the data to see whether the applied change works or not.
• Screenshots or recordings:

Checklist

• I understand every change in this PR. (To the best of my ability 😄)
• I can explain what it does and why it is correct.
• I disclosed any LLM coding help below.

LLM Disclosure

Used AI assistance (Claude) to navigate and study the monorepo and pinpoint where the problem occurs.

[notrevenant]

I was not sure what to title this.