explorer: native Cesium toolbar + prominent share button + a11y halos

[rdhyee]

Summary

Three visible-polish items from Hana's wireframe (Figma 213:394), deferred from #178 as separate from the Light search-semantics work in #179.

• Native Cesium control panel on left edge — re-enable baseLayerPicker (was off) and reposition the Cesium toolbar from default top-right to a vertical strip on the top-left of the globe. baseLayerPicker dropdown opens to the right so it clears the side panel.
• Prominent "Copy Link to Current View" button — #shareBtn restyled full-width with larger padding and 14px font; label updated from "Share View" to match the mockup. Toast moves below the button.
• Accessibility halos — 1.5px white outline on all PointPrimitives in both h3Points (cluster view) and samplePoints (sample view). Universal benefit (contrast on bright land + dark ocean alike), no toggle for v1.

Verified locally

Manual browser eyeball at http://localhost:5880/explorer.html after quarto render:

check	result
Cesium toolbar on left edge of globe, vertical	✓ — 6 widgets stacked (geocoder, home, sceneModePicker, baseLayerPicker, navHelp, fullscreen)
baseLayerPicker dropdown opens to the right	✓ — clears the side panel
Share button label + style	✓ — "Copy Link to Current View", 316px wide, 14px / 10px padding, #1565c0
Halos visible on cluster dots	✓ — orange/blue clusters crisp against both green land and blue ocean
Halos on sample-mode dots	✓ — outline applied at the third point-add site
No console errors (other than benign listings.json 404)	✓

Out of scope (deferred — pending Hana's "corrections coming" revision)

From #178's mockup-derived items list, not in this PR:

• Quick Stats panel restructure (Selected Cluster merged into stats table)
• Permanent sample-rows table at bottom (conflicts with current Globe/Table toggle, needs UX call)
• Tree-selection vocabulary UI (Hana to provide diagram)
• Educational tooltips ("What is a Cluster?", "What is a Sample?") — needs copy
• Top-level sample-type icons (5 per category) — needs assets

Refs #178 (mockup integration umbrella). Companion to #179 (two-button search) which shipped earlier today.

Test plan

• Open Explorer, confirm 6-widget toolbar on left edge of globe
• Click base-layer button — dropdown opens to the right, doesn't clip side panel
• Click home button — globe re-zooms to default view
• Click "Copy Link to Current View" — clipboard receives URL with hash, toast flashes "Link copied!" below the button
• Visually confirm white halos on cluster dots at world zoom and sample dots at street zoom

🤖 Generated with Claude Code