add configurable dot size for zoomed-out structures

Structures collapse to a dot when zoomed out past dotsZoomThreshold.
The dot scale was hardcoded to 1.0 / 2.5 (≈0.4) in structure.vert.glsl.

Promote it to a render setting (`structure.dotScale`) so it's tunable
alongside iconSize / dotsZoomThreshold. Default 0.4 preserves current
behavior. Plumbed via uDotScale uniform on StructurePass.

src/client/render/gl/render-settings.json

@@ -82,9 +82,10 @@
     "railAlpha": 1
   },
   "structure": {
-    "iconSize": 35,
-    "dotsZoomThreshold": 0.5,
-    "iconScaleFactorZoomedOut": 1.4,
+    "iconSize": 65,
+    "dotsZoomThreshold": 1.2,
+    "dotScale": 0.3,
+    "iconScaleFactorZoomedOut": 3.5,
     "shapes": {
       "City": {
         "scale": 1,