[codex] Stabilize explorer map layout

[rdhyee]

Summary

Fixes #199.

This stabilizes the explorer globe layout so the Cesium container is anchored to viewport height instead of deriving its height from grid-column width. It also reserves bounded space for search status text and the view toolbar so loading/search/table controls cannot move the map frame.

Changes

• Replaced #cesiumContainer aspect-ratio sizing with --explorer-map-height: clamp(500px, 65vh, 680px).
• Added a mobile map-height override of clamp(360px, 58vh, 520px).
• Matched .side-panel max height to the same map-height variable.
• Added scrollbar-gutter: stable and minmax(0, 1fr) for layout-width stability.
• Wrapped above-map controls in .explorer-controls, reserved a two-line #searchResults live region, and reserved view-toolbar height so table controls do not shift the table/globe anchor.
• Added Playwright coverage for desktop and mobile layout stability.

Validation

• quarto render explorer.qmd
• node --check tests/playwright/explorer-layout-stability.spec.js
• env NODE_PATH=/Users/raymondyee/C/src/iSamples/isamplesorg.github.io/node_modules /Users/raymondyee/C/src/iSamples/isamplesorg.github.io/node_modules/.bin/playwright test tests/playwright/explorer-layout-stability.spec.js --project=chromium --reporter=line

The focused Playwright spec passed: 2 tests passed on Chromium.