feat(website): store previewed sequence ID in URL

[theosanderson]

resolves #2025

preview URL: http://previewurl.loculus.org

Summary

Stores the ID of the sequence currently previewed in a modal on the search page in the URL

1. Modified both the previewedSeqId and previewHalfScreen state
   to initialize from URL parameters
2. Created callback functions to update both local state and
   URL state
3. Added a useEffect hook to keep the local state in sync with
   URL parameters when they change
4. Used the same pattern as the page parameter:
   - For selected sequence: selectedSeq=ABC123 in URL
   - For half screen mode: halfScreen=true in URL
   - Parameters are completely removed from URL when not active

Now when users:

1. Select a sequence for preview, it will be stored in the URL
2. Change to half-screen view, that preference will be stored
   in the URL
3. Reload the page or share the URL, both the selected sequence
   and view mode will be preserved

Tests aren't working atm though the function does