feat(config, website): multi-seg, multi-ref searchUI mockup

[anna-parker]

resolves #5842, #5945

Screenshot

Screen.Recording.2026-02-04.at.18.16.57.mov

PR Checklist

• All necessary documentation has been adapted.
• The implemented feature is covered by appropriate, automated tests.
• Any manual testing that has been done is documented (i.e. what exactly was tested?)

🚀 Preview: https://search-ui-demo.loculus.org

[claude]

This PR may be related to: #5566 (feature: add option to hide metadata fields from web interface)

[anna-parker]

Comments from @rneher and @theosanderson in https://loculus.slack.com/archives/C094HTT8XEJ/p1770225352248569 (I made issues only for the more complicated sections:

• rename sections to sequence/sample filters
• make subsections more prominent
• "Sample Metadata" should probably be above sequence filters.
• Search UI multi-seg, multi-ref: Remove segment name from mutations in search and on details page #5943
• Search UI multi-path multi-ref: Add custom displayName for segments and references #5944
• Search UI multi-seg multi-ref: Make sequence and sample filter sections collapsable but avoid over-nesting for single segmented organisms #5945
• Rethink displayName of segmented metadata fields on details page

Other issues:

• Make aligned sequence download segment-dependent (now need to select references for all to download any sequence)
• Fix integration tests