Add OpenTopoMap to default basemaps in dft_map_interactive()

Shows stream names, contour lines, roads, and place names for
watershed context alongside satellite and light basemaps.

Fixes #13

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: NewGraphEnvironment

R/dft_map_interactive.R

@@ -79,7 +79,8 @@ dft_map_interactive <- function(x,
                                 titiler_url = getOption("drift.titiler_url"),
                                 basemaps = c("Light" = "CartoDB.Positron",
                                              "Esri Satellite" = "Esri.WorldImagery",
-                                             "Google Satellite" = "https://mt1.google.com/vt/lyrs=s&x={x}&y={y}&z={z}"),
+                                             "Google Satellite" = "https://mt1.google.com/vt/lyrs=s&x={x}&y={y}&z={z}",
+                                             "OpenTopoMap" = "OpenTopoMap"),
                                 legend_position = "bottomright",
                                 zoom = 14) {
   rlang::check_installed(c("leaflet", "leaflet.extras"))