Add preview video section to README (EN + DE)

[BEKO2210]

Embed preview_video.mp4 with HTML video tag and badge-style link in both the English and German sections of the README.

https://claude.ai/code/session_01JWFfVx15Q9Hkx1LLjVKypp

Description

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)
• Performance improvement
• CI/CD changes

Related Issues

Screenshots

Before	After

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my code
• All UI text is in German with correct umlauts (or uses i18n keys)
• I have added translation keys for both de and en if new text was added
• npm run build completes without errors
• npm run test passes all tests
• npx tsc --noEmit reports no type errors
• No unused imports or variables (noUnusedLocals: true)
• Tailwind classes used (no separate CSS files)
• Lucide React icons used (no other icon libraries)

Testing

• Tested in Chrome
• Tested in Firefox/Safari
• Tested on mobile
• Tested offline functionality
• Tested dark mode
• Tested light mode

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