fix(ui): stop panel glows from adding scrollbars to the narrow-screen rails (#699)

At 720px and below both panel stacks scroll vertically, so each panel's absolutely positioned glow (negative inset) became scrollable overflow on both axes and drew a horizontal scrollbar band plus a spurious vertical scrollbar whenever CCTV, Context, Data Layers or Scenes was expanded. Pin the glow to its panel box in the narrow-screen block and add a stylesheet test that checks every 720px media block.

CHANGELOG.md

@@ -1,5 +1,16 @@
 # Changelog
 
+- Remove the spurious scrollbars that appeared on both panel stacks at narrow
+  widths (720px and below) as soon as a panel was expanded. The stacks scroll
+  vertically there, and each panel's decorative glow, absolutely positioned
+  with a negative inset, became 18–20px of scrollable overflow on both axes: a
+  horizontal scrollbar band under the expanded CCTV, Context, Data Layers or
+  Scenes panel plus a vertical scrollbar that scrolled nothing but glow. The
+  narrow-screen rules now pin the glow to its panel box; the stacks still
+  scroll for genuinely tall content such as an expanded DISPLAY panel, and the
+  Context radio popover is not clipped. `src/ui/panelRails.test.mjs` pins the
+  rule against every 720px media block.
+
 - Enable responsive trackpad pinch zoom on the globe. Browser pixel-mode
   `Ctrl+wheel` pinch gestures now reach Cesium with bounded amplification,
   while ordinary wheel, line-mode and touch-pinch inputs retain their existing