feat: add German (de) translation

[molshape]

Description

This PR adds official German translation support to CronMaster.

Changes

• Added app/_translations/de.json with complete German translations
• All UI strings have been translated accurately (except for established idiomatic expressions like "Game Over")
• Translation structure matches the English template app/_translations/en.json

Testing

• Tested with LOCALE=de environment variable
• Verified all pages and components display correctly
• No broken translations or missing keys

Checklist

• Translation is complete (all keys translated)
• No JSON syntax errors
• Follows translation guidelines
• Tested in development environment
• Commit message follows conventional format

Additional Information

• Uses neutral, pronoun‑free phrasing to avoid formal/informal ambiguity in UI texts
• Applies consistent German IT terminology (e.g., “Backup” → “Sicherung”, “Task” → “Aufgabe”)
• Adapts confirmation prompts to natural German phrasing instead of literal translations (e.g., "Are you sure…" → "Soll ... wirklich ...")
• Ensures consistent terminology across all sections (e.g. "Log" → "Protokoll")
• No translation of established idiomatic expressions (e.g., "GAME OVER", "Job", or "Exit-Codes")
• All keys from en.json are present in de.json
• German special characters (Umlaute and ß) render correctly in the UI and were verified during testing
• Follows translation guidelines and structure outlined in howto/TRANSLATIONS.md