Add privacy notice and imprint footer links with full backend support

[jhaals]

• Add backend configuration flags --privacy-notice-url and --imprint-url
• Update server config endpoint to include privacy/imprint URLs when configured
• Add conditional footer links for privacy notice and imprint URLs in frontend
• Include translations in EN, SV, and DE locales
• Links only display when URLs are configured and non-empty
• Proper external link attributes (target="_blank", rel="noopener noreferrer")
• Update TypeScript interface with footer and buttonText types
• Add comprehensive Playwright tests for footer link behavior
• Update README documentation with new command-line flags

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.29%. Comparing base (c0302dc) to head (ac5a71d).
⚠️ Report is 6 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3012      +/-   ##
==========================================
+ Coverage   83.09%   83.29%   +0.19%     
==========================================
  Files           8        8              
  Lines         781      790       +9     
==========================================
+ Hits          649      658       +9     
  Misses         98       98              
  Partials       34       34              

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