chore(fe): fix German Undo translation per reviewer feedback (#3769)

# Motivation

The "Undo" button was translated as "Rückgängig" in German, which is
incomplete as a standalone button label. Per reviewer feedback from
@marc0olo on #3743, this updates it to "Rückgängig machen" — the
standard German UI convention for undo actions.

# Changes

- Updated the German translation for "Undo" from "Rückgängig" to
"Rückgängig machen" in `de.po`.

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: sea-snake

src/frontend/src/lib/locales/de.po

@@ -1004,7 +1004,7 @@ msgid "Understanding Internet Identity"
 msgstr "Internet Identity verstehen"
 
 msgid "Undo"
-msgstr "Rückgängig"
+msgstr "Rückgängig machen"
 
 msgid "Unexpected error"
 msgstr "Unerwarteter Fehler"