NRF2-418 Added OS base map

[robin-dunn]

• Add Ordnance Survey Vector Tile API as the base map layer on the /quote/map page, replacing the MapLibre
  demo tiles
• Create a server-side proxy (/base-map) that forwards tile requests to the OS API, keeping the API key out
  of the browser and avoiding CORS issues
• The proxy rewrites URLs in style/TileJSON responses so all map resource requests (tiles, sprites, glyphs)
  route through our server
• Cap fitBounds zoom to level 15 to match the OS VTS maximum zoom

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
98.4% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud