fix(lanes): make Add Lane mode segments fill their row and read as selected

The two-way segment used a translucent accent wash for the active state,
which at this size read as a hover tint rather than a selection. Solid
accent plus a shadow makes the choice unambiguous, and `flex-1` stops the
two segments from sizing to their label text.

client/src/components/lanes/AddLaneModal.tsx

@@ -60,8 +60,10 @@ function Seg({
       onClick={onClick}
       title={title}
       aria-pressed={active}
-      className={`rounded px-2 py-0.5 text-xs font-medium transition-colors ${
-        active ? "bg-accent/20 text-accent" : "text-fg-secondary hover:text-fg-primary"
+      className={`flex-1 rounded px-2 py-1 text-xs font-medium transition-colors ${
+        active
+          ? "bg-accent text-white shadow-sm"
+          : "text-fg-secondary hover:bg-surface-3 hover:text-fg-primary"
       }`}
     >
       {label}