feat(tours): plan ski and winter tours, snowshoeing and winter hiking among them

Ski becomes "Ski & winter", a type the planner plans with six disciplines
(alpine, snowboard, cross-country, ski touring, snowshoeing, winter hiking),
nothing preselected; saving needs a discipline, and a technique for
cross-country. Every leg is drawn straight between the points through
planner/skiRouting.ts, the one place the piste router will plug in, with its
heights from the elevation source and no ski time worked out.

The hike's winter label goes entirely: its migration, column, field, marks,
GPX rule, MCP field and strings. A hike is summer only; a ski tour with the
snowshoe discipline exports as snowshoeing. The unreleased ski migration takes
the six disciplines and the collection migration loses the winter column.

The pistes are offered for a ski tour, shown until switched off, and with no
tour open, never for a hike. Export, copy, collections, stages (untimed),
actual tracks, the atlas, list and day marks (one glyph per discipline), 3D,
planning over a ski import and list_tours/get_tour_plan know the planned ski
tour.

client/src/components/Map/MapLayerSwitcher.tsx

@@ -209,7 +209,7 @@ interface OverlayLook {
 
 const OVERLAY_LOOK: Record<string, OverlayLook> = {
   seamarks: { Icon: Anchor, labelKey: 'tours.map.seamarks', downKey: 'tours.map.seamarksDown' },
-  // SKI-01: a hike's row already holds four maps, the ferratas and 3D, so the pistes go by their icon.
+  // SKI-01: the row already holds the maps and 3D, so the pistes go by their icon.
   pistes: {
     Icon: MountainSnow,
     labelKey: 'tours.map.pistes',