feat(onboarding): add German locale

[13robin37]

Summary

Adds a complete German (de) catalog for Secure Dynamic Onboarding and embeds it into secure_onboarding.py via sync_locales.py.

• 561/561 messages translated, informal address ("du"), matching the tone of the Open WebUI German UI
• _meta: de / German / Deutsch
• Placeholders, leading/trailing spaces, @, #, /, +, ›, ⋯ and emoji preserved
• Avatar initial for "Your name" is D (Dein Name), analogous to V in French

Interface labels

The locale README asks to keep product labels unchanged. A user who gets the German guide has Open WebUI set to German, so the UI shows German labels. I therefore used the exact strings from the official Open WebUI de-DE translation (e.g. "Werkzeug-Berechtigungen", "Wissen anhängen", "Admin-Bereich", "Einstellungen › Personalisierung › Erinnerungen").

Labels that have no German translation upstream are left in English because that is what the German UI displays: "Ask for approval", "Full access", "New Event", "New Automation", "Execution logs".

Happy to switch to English labels (like fr.json) if you prefer consistency across catalogs.

Not included

README and CHANGELOG language lists are left untouched, as you updated them yourself for Spanish and Catalan.

Checks

python sync_locales.py          → synchronized 561 messages across 5 locales
python sync_locales.py --check  → validated 561 messages across 5 locales
python -m unittest -v test_secure_onboarding.py → Ran 21 tests, OK