[v2 alpha] devtools are always light mode

using `formDevtoolsPlugin()`, devtools are always light mode even if `TanStackDevtools` says dark.

see the query devtools in this screenshot for comparison:

<img width="1713" height="433" alt="Image" src="https://github.com/user-attachments/assets/017dbd34-03e4-4321-abb5-4106a5a8697b" />

using:

```
"@tanstack/react-devtools": "0.10.12",
"@tanstack/react-form-devtools": "1.0.0-alpha.2",
```