[Platform]: Add sticky profile header with widget navigator

[carcruz]

Description

GitHub PR-style sticky bar for target/disease/drug/variant/study/credible-set profile pages: shows the entity title/icon/external references plus a dropdown (with filter) listing every widget on the page, so long profiles (target has 18 widgets) get a quick jump menu instead of just scrolling.

• StickyProfileHeader (packages/ui): fixed-position bar, reveals once the Profile/Associated tabs row scrolls out of view (not always-on), full viewport width, no shadow. Reuses each widget's hasData()/shortName the same way the existing Summary cards do, so disabled/empty widgets are greyed out consistently. Active widget label crossfades in on change.
• useActiveSection: IntersectionObserver-based scrollspy reporting which widget sits in the upper-middle of the viewport. Section bodies are React.lazy/Suspense-loaded, so a MutationObserver rescans for their DOM nodes as they mount.
• URL sync: active widget id syncs to the URL hash while scrolling (raw history.replaceState — fires often, doesn't need a real navigation), while explicit jumps (dropdown pick, Summary card click) use navigate() with replace+preventScrollReset, since those are one-off user actions where going through the router is correct. useScrollToHashOnMount replays a deep-linked hash on load once the lazy-loaded section exists.
• Icon/externalLinks lifted out of each entity's Header.tsx into an exported buildHeaderMeta(), so the sticky bar and the full-size header render from one source of truth instead of duplicating the xref-building logic.
• Bug fix: BaselineExpression's tab body had a key={crypto.randomUUID()} that force-remounted (losing tab/chart state) on every render — unrelated pre-existing bug, surfaced while testing the scroll-driven URL sync.

Issue: —
Deploy preview: —

Type of change

• New feature (non-breaking change which adds functionality)
• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

• yarn turbo run build --filter=platform after each change, no new type errors
• Manual verification in-browser across all 6 profile page types: sticky reveal timing, full-width layout, dropdown filter/greying, active-section tracking while scrolling, hash deep-linking (copy URL mid-scroll into a fresh tab), BaselineExpression no longer resetting on scroll

Checklist:

• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I have made corresponding changes to the documentation