Add German translations for Downloads and FAQ pages
- downloads-DE.md: Complete German translation of Downloads page with platform-specific installation instructions (Windows, macOS, Linux) and troubleshooting
- faq-DE.md: Comprehensive German FAQ covering installation, getting started, creating/saving diagrams, project organization, and elements/libraries
- languages.md: Updated translation status matrix showing Downloads and FAQ now complete for all three languages (English, Français, Deutsch)
Completes Option A expansion of multi-language support for QET wiki.
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