chore(deps): group react and react-dom in dependabot config

React enforces exact version parity between react and react-dom.
Bumping them independently causes build failures. Grouping ensures
Dependabot always updates them together in a single PR.

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

Author: sserrata

.github/dependabot.yml

@@ -4,3 +4,8 @@ updates:
     directory: /
     schedule:
       interval: daily
+    groups:
+      react:
+        patterns:
+          - "react"
+          - "react-dom"