Resolve "Reenable Katex: Was disabled because rendering problems"

Related to #593 (closed)

Summary by CodeRabbit

• New Features

  • Validation UI gains "Repair" and "Send Mails" options; automatic repair can create exercise metadata from a new template; KaTeX stylesheet added for math rendering.

• Improvements

  • Validation now detects project types; index UI shows modification info and auto-scrolls logs; quieter CI download output; broadcast messages include a theme field; multiple i18n label updates.

• Bug Fixes

  • Removed duplicate build entries.

• Dependency Updates

  • Updated generator-jhipster and i18n-check; added Thymeleaf starter.

• Tests

  • New integration test and test data for GitLab event handling.