fix(maps): draw with Leaflet wherever WebGL2 is missing

MapLibre 6 and Mapbox GL 3 both throw out of their Map constructor without
WebGL2. Every place that builds a GL map now asks one helper first,
components/Map/glSupport.ts, and a browser without WebGL2 gets the Leaflet
map with the user's tiles instead of a map that fails to start:

- the trip map (MapViewAuto) and the journey maps (JourneyMapAuto) pick
  their renderer through useGlRenderer, which folds in the Mapbox token
  rule they each had;
- the settings preview, desktop and phone, shows the Leaflet preview
  through useMapSettingsForm and says why under it;
- the tour planner keeps the 2D map when 3D is asked for and says that
  3D needs WebGL2;
- the PDF route map and the Leaflet vector basemap already fell back and
  now report a GPU failure too.

Behind the probe, buildGlMap wraps each constructor (MapViewGL,
JourneyMapGL, the settings preview, the terrain map, the PDF map). A GPU
that claims WebGL2 and still refuses the context is reported through
utils/webgl, which turns the answer to no and tells every mounted map,
so they switch to Leaflet without a reload; any other error is still
thrown. Two new strings, map.webgl2.fallback and map.webgl2.terrain, in
all 27 locales.

client/src/components/Map/MapViewGL.tsx

@@ -24,6 +24,7 @@ import { useTransportRoutes } from '../../hooks/useTransportRoutes'
 import { visibleRouteReservations } from '../../utils/reservationRoutes'
 import { safeHexColor } from '../../utils/safeColor'
 import { MAPBOX_DEFAULT_STYLE, styleForActiveProvider, basemapLanguage, type GlMapProvider } from './glProviders'
+import { buildGlMap } from './glSupport'
 import LocationButton from './LocationButton'
 import { useIsPhone } from '../../mobile/useIsPhone'
 import { useGeolocation } from '../../hooks/useGeolocation'
@@ -1104,13 +1105,12 @@ export function MapViewGL({
       antialias: mapboxQuality,
     }
     if (!isMapLibre) mapOptions.projection = mapboxQuality ? 'globe' : 'mercator'
-    // MapLibre 5's mouse-rotate inverts its sign at a mid-screen line it gets by
-    // re-projecting the map center — a line that drifts with the bearing, so a
-    // right-button drag near mid-screen ping-pongs instead of rotating (#1545).
-    // aroundCenter: false restores the plain dx-based rotate mapbox-gl uses.
+    // MapLibre's around-center rotate ping-pongs near mid-screen (#1545); false is mapbox-gl's dx rotate.
     if (isMapLibre) mapOptions.aroundCenter = false
 
-    const map = new gl.Map(mapOptions as any)
+    // Null when the GPU refused WebGL2: MapViewAuto has switched to Leaflet by then.
+    const map = buildGlMap(() => new gl.Map(mapOptions as any))
+    if (!map) return
     mapRef.current = map
     popupRef.current = new gl.Popup({
       closeButton: false,