Add a Hiking trails overlay (Waymarked Trails, online only)

New Overlays panel toggle, same plain on/off shape as Contour lines —
raster tiles from waymarkedtrails.org, which already color each route
by its OSM colour tag (the KST/KČT paint-blaze colors used in
Slovakia/Czechia, plain red elsewhere), so no styling work needed here.

Online only for now; offline (baked into offline.pmtiles) needs a
custom Planetiler profile for route relations, deferred until there's
a route planner/router to pair it with.

views/layout.erb

@@ -35,6 +35,7 @@
         "/js/dem.js": "/js/dem.js?v=<%= ASSET_VERSION %>",
         "/js/gnss.js": "/js/gnss.js?v=<%= ASSET_VERSION %>",
         "/js/gpx.js": "/js/gpx.js?v=<%= ASSET_VERSION %>",
+        "/js/hiking_trails.js": "/js/hiking_trails.js?v=<%= ASSET_VERSION %>",
         "/js/icons.js": "/js/icons.js?v=<%= ASSET_VERSION %>",
         "/js/livetrack.js": "/js/livetrack.js?v=<%= ASSET_VERSION %>",
         "/js/locate.js": "/js/locate.js?v=<%= ASSET_VERSION %>",