fix: avoid loading dev toolbar CSS in production

[sean0407]

Summary

ErrorBoundary
  when the development overlay is enabled -> clientOnly(import DevToolbar)
  otherwise -> the default error boundary

The static DevToolbar import pulled its stylesheet into production client pages even when the overlay was disabled. This reuses SolidStart's existing lazy clientOnly loader.

Fixes #2327.

Evidence

• Before: With the fixture app's own CSS import removed, a production build and GET / still linked /_build/assets/entry-client-EBetCt96.css (16,856 bytes; 3.24 kB gzip).
• After: The same production fixture no longer links an entry-client or dev-toolbar stylesheet. The toolbar CSS is emitted as a separate lazy asset.
• Production fixture build succeeded, and oxfmt --check passed for the changed source and changeset.

Merge Danger

Door: Two-way

Blast Radius: Low

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🦋 Changeset detected

Latest commit: f5f73ce

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@solidjs/start	Patch

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