mapLibre.ts - stop the popups from moving around when panning

MapLibre pop-ups by default try to resist going off-screen when
panned.  This causes a bit of havoc when small-screen users try to
pan, because the pop-up jumps around unpredicably.

But if we specify where to put the anchor, the pop-up will stick with
that and stay still with respect to the tiling.

apps/front-end/src/components/map/mapLibre.ts

@@ -58,6 +58,7 @@ const openPopup = async (
   popup = new Popup({
     closeButton: false,
     maxWidth: "none",
+    anchor: "bottom",
   })
     .setLngLat(coordinates)
     .setHTML(`<div id=${POPUP_CONTAINER_ID}></div>`)