ci: pin the pnpm version in the e2e workflow (#261)

The v2.9.0 release run's e2e gate died at startup of the visual job:
pnpm/action-setup@v4 with no version input requires a root package.json
packageManager field, and ours lives in ui/. e2e.yml is
dispatch/call-only, so the release gate was its first real CI execution.
This pins @v6 + version: 10, matching every sibling workflow. After
merge the v2.9.0 tag will be re-pointed at the fixed commit to re-fire
the release build.

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

https://claude.ai/code/session_015jEKvjBzVjaWLq6o5zg2PV

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

Author: pmaxhogan

.github/workflows/e2e.yml

@@ -72,7 +72,9 @@ jobs:
     steps:
       - uses: actions/checkout@v7
 
-      - uses: pnpm/action-setup@v4
+      - uses: pnpm/action-setup@v6
+        with:
+          version: 10
 
       - uses: actions/setup-node@v7
         with: