i18n: complete German (de-DE) translations (#26383)
Translate all remaining untranslated strings in the German locale: 228 empty values plus the genuinely translatable members of the value-equals-key set (240 entries total). Covers webhooks/events, external knowledge sources, context compaction, terminal servers, OAuth/OIDC claims, skills, and directory management. Translations follow the existing locale conventions: formal "Sie" address, established German terms (Websuche, Wissen, Werkzeuge, Wissensspeicher), and brand/proper nouns, format names, and identical-in-German words left untouched.
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