fix(ci): pin e2e-chat setup-node to mirrored SHA (mc#1292)

actions/setup-node@60edb5dd... (v4) was never mirrored into the
self-hosted Gitea Actions mirror, causing 100% failure rate for E2E
Chat since inception (33 runs, 0 successes).

Switch to the already-mirrored v6.4.0 SHA that sibling workflow
e2e-staging-canvas.yml uses successfully.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

.gitea/workflows/e2e-chat.yml

@@ -97,7 +97,7 @@ jobs:
           cache-dependency-path: workspace-server/go.sum
 
       - if: needs.detect-changes.outputs.chat == 'true'
-        uses: actions/setup-node@60edb5dd545a775178f52524783378180af0d6f5 # v4
+        uses: actions/setup-node@48b55a011bda9f5d6aeb4c2d9c7362e8dae4041e # v6.4.0 (mirrored SHA; v4 SHA was never mirrored — mc#1292)
         with:
           node-version: '22'
           cache: 'npm'