Routing: Highlight waypoints in the map

[dopenguin]

Description

A user should be able to track where added waypoints are located in the map.
To avoid confusion with existing map features, the waypoint icon should be visually distinct from the markers/pins styling e.g. through a flag icon.

Current behavior

After a user selects an input and clicks in the map, the only feedback he receives is the updated input. He does not see, where exactly his waypoint has been added.

Expected behavior

A small icon should be placed at the position the user's selected waypoint is added. If a user selects a waypoint by entering an address (see #865), the icon should still be added.

• Each entry in route that has a coordinate is rendered as a point marker on the map at that position.
• The marker is added/updated/removed reactively as waypoints are added, changed, reordered, or removed.
  Markers appear consistently whether the waypoint was set via map click or via address search (Routing: Search for addresses #865).

Affected areas

File	What needs to change
src/plugins/routing/store.ts	Add a dedicated vector source/layer for waypoint point features; check Material Symbols for a possible icon or consult with our usability expert regarding a possible solution. Keep it in sync with route e.g. via the existing route watch and clean it up in teardownPlugin/reset.

Out of scope

This feature only focuses on the visual addition of the icon at any waypoint. No interaction on the marker itself (e.g. click, hover, drag to move) is part of this issue.