Set GitHub Sponsors username in FUNDING.yml

[patrickschiller]

Updated GitHub funding model to include a specific username.

Summary

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Related issue

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Validation

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Checklist

• The change is focused and does not include private archive content,
  secrets, personal data, or unrelated files.
• I used synthetic or redistributable fixtures and screenshots.
• I added or updated tests for behavioral changes.
• Relevant tests pass, or I documented why a check could not run.
• User-facing English and German documentation stay consistent.
• Every commit includes a DCO Signed-off-by: trailer (git commit -s).
• My contribution is licensed under the project's MIT License.