Extend heritage search filters with Endangered + UNESCO Criteria

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Summary

Review feedback on the current heritage search form: it does not let users filter by endangered status (is_endangered) or UNESCO criteria (i–x). This issue tracks the design decisions,
child-issue breakdown, and follow-up UX work needed to land both filters cleanly.

Background

• The form currently exposes Region, Category, Keyword, and Year-inscribed range only.
• WorldHeritageVm already carries isEndangered and criteria, so display is unblocked — only the search filter side is missing.
• Criteria is multi-valued, which introduces structural decisions (URL encoding, AND vs OR) that the existing single-select filters never had to make. Endangered is a single boolean and is closer to a
  straight add.

Decisions needed before child work

• Backend readiness: confirm is_endangered and criteria[] query params are supported by the API (or schedule a backend change)
• Criteria semantics: AND (sites must satisfy all selected criteria) or OR (any selected criteria)? Default proposal: AND
• Multi-value URL encoding: comma-separated (criteria=ii,iv) vs repeated keys (criteria=ii&criteria=iv). Default proposal: comma-separated
• Endangered shape: 2-state checkbox ("Endangered only", off = no filter) vs 3-state chip ("All / Endangered / Safe"). Default proposal: 2-state