Fix Vue node selection detection in browser tests (#5555)

The VueNodeHelpers was using incorrect CSS selector for detecting selected nodes.
Vue nodes use outline-black/outline-white classes for selection state, not border-blue-500.

This fixes the failing delete key interaction tests that were showing 0 selected nodes
when they should have been detecting the actual selection state.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-authored-by: Claude <noreply@anthropic.com>

browser_tests/fixtures/VueNodeHelpers.ts

@@ -17,7 +17,9 @@ export class VueNodeHelpers {
    * Get locator for selected Vue node components (using visual selection indicators)
    */
   get selectedNodes(): Locator {
-    return this.page.locator('[data-node-id].border-blue-500')
+    return this.page.locator(
+      '[data-node-id].outline-black, [data-node-id].outline-white'
+    )
   }
 
   /**