style(graph): use graph-supplied lane color for behind segments

Remove hardcoded stroke and drop-shadow for "behind" graph segments.
This allows the lane color provided by the graph to determine the stroke.
Keep the dashed stroke pattern so "behind" status remains visually clear.

- Remove explicit stroke and filter for behind segments
- Add clarifying comment and retain dash pattern

src/app.css

@@ -3368,10 +3368,9 @@
     stroke: var(--color-sync-ahead);
     filter: drop-shadow(0 0 3px color-mix(in srgb, var(--color-sync-ahead) 24%, transparent));
   }
+  /* Keep the lane color supplied by the graph; dashes indicate behind status. */
   .graph-svg path.graph-segment-behind {
-    stroke: var(--color-sync-behind);
     stroke-dasharray: 4 4;
-    filter: drop-shadow(0 0 3px color-mix(in srgb, var(--color-sync-behind) 20%, transparent));
   }
   .graph-svg path.graph-ref-connector {
     opacity: 0.35;