React frontend

A magyar leírás lent található

## English description

By now the app's frontend got too complex for the erb templates. So the next logical thing to do is to move to a separate fontend. This migration should be done incrementally starting with simple pages then gradually replacing everything.

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## Magyar leírás

Mára a frontend elérte azt a komplexitást, hogy ideje átállni szeparált frontend-backend architektúrára. Ezt az átállást inkrementálisan, az egyszerű oldalakkal kezdve kell megtenni.

The issue describes an incremental migration away from ERB templates but names no files, frontend technology, entry point, or tests. Start by mapping the existing ERB frontend and current Rails structure, then identify a small simple page as a migration boundary. Done would require an agreed frontend architecture and a completed incremental migration, which the issue does not define.

Written by the indexing model from the issue text.