Add world coordinates to regiment exports for GIS integration

Regiment CSV Export Enhancements:
- Add X_World (m) and Y_World (m) columns using meters per pixel conversion
- Include both current and base position world coordinates
- Rename existing coordinate columns to X_Pixel/Y_Pixel for clarity
- Add getMetersPerPixel() helper function supporting km, m, and miles units

GeoJSON Regiment Export:
- Create new saveGeoJsonRegiments() function in export.js
- Export regiments as Point features with Fantasy Map Cartesian coordinates
- Include all military unit data, state information, and position metadata
- Add regiments button to GeoJSON export UI section

This enables direct import of regiment data into QGIS using the custom
Fantasy Map Cartesian CRS with proper world coordinate positioning.

index.html

@@ -6052,6 +6052,7 @@
           <button onclick="saveGeoJsonRivers()" data-tip="Download rivers data in GeoJSON format">rivers</button>
           <button onclick="saveGeoJsonMarkers()" data-tip="Download markers data in GeoJSON format">markers</button>
           <button onclick="saveGeoJsonBurgs()" data-tip="Download burgs data in GeoJSON format">burgs</button>
+          <button onclick="saveGeoJsonRegiments()" data-tip="Download regiments data in GeoJSON format">regiments</button>
         </div>
         <p>
           GeoJSON format is used in GIS tools such as QGIS. Check out