CLAUDE.md: names in code and tests are in English

A new bullet, placed right after the one on UI strings (which already covers the German and English string tables). Nothing else changes.

- UI strings live in one place, `src/ui/strings.ts`, in German and English (two tables of one shape).
  German UI texts and docs call a button "Button" or "Schaltfläche", never "Knopf". Docs written in
  English name UI elements by their labels in the English table, never by the German ones.
- Names in code and tests (identifiers, fixture and fake names) are in English, so English-speaking
  contributors can follow them; German belongs only in the German UI string table and in test data
  that has to be German.