add warning for open registration

[syuilo]

What

Why

Additional info (optional)

Checklist

• Read the contribution guide
• Test working in a local environment
• (If needed) Add story of storybook
• (If needed) Update CHANGELOG.md
• (If possible) Add tests

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 17 lines in your changes missing coverage. Please review.

Project coverage is 19.33%. Comparing base (e26e24b) to head (7c48018).
Report is 12 commits behind head on develop.

Files with missing lines	Patch %	Lines
packages/frontend/src/pages/admin/moderation.vue	0.00%	17 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           develop   #14963       +/-   ##
============================================
- Coverage    41.62%   19.33%   -22.29%     
============================================
  Files         1564      727      -837     
  Lines       202872   103616    -99256     
  Branches      3708      990     -2718     
============================================
- Hits         84440    20033    -64407     
+ Misses      117825    83030    -34795     
+ Partials       607      553       -54     

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[syuilo]

🙏🏻