fix: dismiss branch hover card when mouse leaves to scrim

The fixed scrim was catching the mouse, keeping the hover alive
indefinitely. Now the scrim has onMouseEnter={() => setIsHovered(false)}
so moving the mouse anywhere off the expanded card dismisses it.

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

frontend/src/components/session/BranchNode.tsx

@@ -97,8 +97,11 @@ export function BranchNode({ branch, depth, isActive, onClick }: BranchNodeProps
       {/* Expanded card — floats directly on top of the base card */}
       {showExpanded && (
         <>
-          {/* Scrim behind to dim everything */}
-          <div className="fixed inset-0 z-40 bg-black/30" />
+          {/* Scrim behind to dim everything — dismisses on mouse enter */}
+          <div
+            className="fixed inset-0 z-40 bg-black/30"
+            onMouseEnter={() => setIsHovered(false)}
+          />
 
           {/* Expanded card positioned over the original */}
           <div