perf: remove redundant CSS opacity transition from all nodes

The `.react-flow__node:not(.switch-dimmed)` rule applied a transition to
every node DOM element unnecessarily. The `.switch-dimmed` rule already
handles the transition for dimming. Removing this makes undimming instant
(snappier) and eliminates a transition declaration from every node.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/app/globals.css

@@ -55,10 +55,6 @@ body {
   transition: opacity 250ms ease-in-out;
 }
 
-.react-flow__node:not(.switch-dimmed) {
-  transition: opacity 250ms ease-in-out;
-}
-
 /* Base handle styles */
 .react-flow__handle {
   width: 14px;