Default basemap to OSM when no Carto API key is configured

[Yeraze]

Summary

When no Carto API key is configured, MeshMonitor still defaults to a Carto basemap (Dark Matter or Voyager). Since Carto now requires an API key for their tile endpoints, new installs see tiles covered in "API KEY REQUIRED" watermarks instead of a usable map.

Expected Behavior

When no Carto API key is present, MeshMonitor should default to OpenStreetMap (which requires no API key) so new users see a working map out of the box.

Current Behavior

New installs load Carto tiles that render with diagonal "API KEY REQUIRED — carto.com/basemaps/apikey" watermarks across the entire map. The map is functional (markers/clusters still render) but the basemap is effectively unusable.

Suggested Fix

• Default the basemap to OSM when no Carto API key is configured
• Optionally show a settings warning or toast when a Carto style is selected but no API key is set

Screenshot

User-reported screenshot shows the Carto Dark Matter basemap with repeating "API KEY REQUIRED" watermark text across all tiles.

Context

• Carto API key support was added in PR feat(map): support a Carto basemap API key (#4934 Phase 1) #4936 / fix(settings): move Carto API key field to the Map section #4938 (v4.15.2-rc5)
• Reported by Zeryl in Discord #meshtastic on 2026-09-01

Authored by NodeZero 0️⃣

[Yeraze]

Update from testing: Zeryl confirmed this only happens in dark mode. MeshMonitor appears to follow the browser/OS dark mode preference and defaults to a Carto dark tileset when in dark mode. Light mode defaults to OSM correctly.

So the fix scope is narrower: the dark mode default basemap should fall back to OSM (or another keyless dark style) when no Carto API key is configured.

Authored by NodeZero 0️⃣