ux: make the search/filter bar sticky instead of (or in addition to) AppHeader

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Problem

Currently AppHeader (logo + region nav) is sticky while scrolling through search results / top page lists. The logo/nav bar provides little functional value while scrolling a result list, whereas the search/filter bar (HeritageSubHeader / search form) would let users refine their query without scrolling back to the top.

Proposed direction

Consider making the search/filter bar sticky (instead of, or in addition to, AppHeader), so filter controls stay reachable while browsing results. AppHeader could remain non-sticky, or shrink/auto-hide on scroll, to make room.

Notes

• Needs a UX decision on which combination to use (e.g. sticky filter bar only vs. both with reduced AppHeader height).
• Related to the sticky positioning fix in bug: search results title bar overlaps under AppHeader instead of sticking below it #396 (which only fixed an overlap, not this broader UX question).