[feature] Migrate basemap provider for genuine colorful light+dark tiles

[AnayDhawan]

What problem does this solve?

The map basemap (CARTO Voyager/dark_all) reads as muted/colorless,
especially in dark mode. CARTO's only colorful style is Voyager, and it's
light-only — there's no colorful dark equivalent in CARTO's catalog, so a
CSS saturate filter has a hard ceiling: you can't saturate a colour that
isn't there.

Proposed solution

Migrate to MapTiler Cloud: free tier (100k tiles/month, no credit card),
genuine theme-aware light+dark colorful raster styles, drop-in Leaflet
TileLayer URL swap. Keep the existing useTheme()-driven swap pattern
(key={tileVariant} remount) in src/components/map/map-view.tsx. Needs
NEXT_PUBLIC_MAPTILER_KEY set locally (.env.local) and in Vercel project
env vars.

Alternatives considered (optional)

• Plain OpenStreetMap tiles: no signup needed, colorful in light mode, but
  no native dark style and OSM's usage policy isn't meant for sustained
  production traffic.
• Stadia Maps / Jawg: also viable, lower free-tier quotas than MapTiler.

Checklist

• I searched existing issues and this is not a duplicate.