Re-add Allmaps viewing to the Geoportal

[ewlarson]

Summary

Closes #72.

This re-adds Allmaps support to the new Geoportal resource page:

• adds Item Viewer / Map Overlay tabs when a resource has meta.ui.allmaps
• renders an Allmaps georeferenced overlay with @allmaps/leaflet, basemap controls, and opacity control
• adds a sidebar Map Overlay card with links to the Allmaps viewer and editor
• centralizes Allmaps URL generation in a small frontend utility
• covers the tabs, sidebar links, and overlay rendering path in ResourceView tests

Validation

• npm test -- ResourceView.test.tsx
• ./node_modules/.bin/eslint src/pages/ResourceView.tsx src/components/resource/AllmapsOverlayViewer.tsx src/components/resource/AllmapsOverlayViewer.client.tsx src/components/resource/AllmapsLinksCard.tsx src/utils/allmaps.ts src/__tests__/components/ResourceView.test.tsx
• ./node_modules/.bin/tsc --noEmit -p tsconfig.json
• npm run build
• visually checked p16022coll230:4038 in a production preview against the dev API; the Map Overlay tab rendered the Allmaps layer and the sidebar viewer/editor links were present