Add {bbox} and {bbox_3857} URL substitution for WMS proxy support

[catapulta]

Summary

Adds {bbox} and {bbox_3857} URL substitution to the tile proxy so WMS chart sources can be proxied and cached alongside regular tile sources.

NOAA retired their RNC tile service and the Chart Display Service now serves S-57 ENC data through WMS only — no XYZ tile endpoint. This makes caching impossible with the current proxy since it only does {z}/{x}/{y} substitution.

Addresses #53

What changed

• {bbox} replaced with minLon,minLat,maxLon,maxLat (EPSG:4326)
• {bbox_3857} replaced with the same bounds in Web Mercator
• tileToBBox promoted from private to static (already existed, just needed to be callable from the static fetchTileFromRemote)
• Small lonLatToMercator helper for the 3857 projection

No changes to config schema, proxy routes, or caching/seeding — those already work on tiles internally. Fully backwards compatible: the bbox logic only fires if the URL contains {bbox} or {bbox_3857}, existing configs are untouched.

Example: NOAA S-57 ENC with caching

Field	Value
Server type	tilelayer
Format	png
Proxy	enabled
URL	https://gis.charttools.noaa.gov/arcgis/rest/services/MCS/NOAAChartDisplay/MapServer/exts/MaritimeChartService/WMSServer?service=WMS&version=1.3.0&request=GetMap&layers=0,1,2,3,4,5,6,7,8,9,10,11,12&styles=&crs=EPSG:3857&bbox={bbox_3857}&width=256&height=256&format=image/png&transparent=true

Testing

• Tested on Raspberry Pi 4 running OpenPlotter / SignalK 2.19.1 with Freeboard-SK
• NOAA ENC tiles fetch, cache to disk, and serve from cache on subsequent requests
• Existing {z}/{x}/{y} tile sources continue to work unchanged