fix: make disabled route control buttons visually distinct

Disabled buttons now show reduced opacity (0.35) and muted icon
color instead of just changing the cursor.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/src/pages/Exchanges/RouteControlBar.module.css

@@ -52,6 +52,12 @@
 
 .segment:disabled {
   cursor: not-allowed;
+  opacity: 0.35;
+  color: var(--text-muted);
+}
+
+.segment:disabled svg {
+  color: var(--text-muted) !important;
 }
 
 .divider {