fix(journey): keep GL photo markers absolutely positioned so thumbnails sit on their coordinates

The journey map's photo thumbnails set position:relative inline on the marker
element. That overrides the engine's .mapboxgl-marker/.maplibregl-marker
position:absolute, so every thumbnail entered the normal flow below the ones
drawn before it and the engine's translate was added on top: the further down
the list, the further the photo drifted from its spot. The location dot had the
same inline position. Both now leave position to the engine's class.

Closes #2910

client/src/components/Map/locationMarkerMapbox.ts

@@ -13,7 +13,9 @@ type MarkerConstructor = new (options?: { element?: HTMLElement; anchor?: string
 // and mapbox doesn't get confused about which element to position.
 function buildLocationEl(): { root: HTMLDivElement; cone: HTMLDivElement } {
   const root = document.createElement('div')
-  root.style.cssText = 'width:28px;height:28px;position:relative;pointer-events:none;'
+  // No inline `position`: the GL marker class keeps the root absolute, which the
+  // children's insets resolve against (see the note on markers in MapViewGL).
+  root.style.cssText = 'width:28px;height:28px;pointer-events:none;'
   // Accuracy pulse behind the dot
   const pulse = document.createElement('div')
   pulse.style.cssText = `