[janhelcl]

Summary

• use CARTO's keyed raster endpoint for lightweight date-thumbnail maps
• fall back to the existing OpenStreetMap tile layer when no CARTO key is configured
• show shared OpenStreetMap/CARTO attribution beneath the thumbnail strip
• expose VITE_CARTO_API_KEY / REACT_APP_CARTO_API_KEY through the Vite build
• document local and production configuration and add unit coverage

Why

CARTO began requiring API keys for raster basemaps in August 2026. The main map was unaffected because it uses OpenStreetMap/OpenTopoMap, while the date thumbnails still used an unauthenticated CARTO URL.

Verification

• npm test: 16 files, 42 tests passed
• npm run build: client and SSR builds passed
• client build with a fake VITE_CARTO_API_KEY confirmed the value reaches the bundle
• git diff --check: passed

Deployment

The fallback removes the watermark without configuration. To retain the lighter CARTO thumbnail style, add a domain-restricted VITE_CARTO_API_KEY to the frontend build environment.