fix(workbenches): preserve host stylesheet ownership

[cinderzhan]

Summary

• Mark the Desktop workbench stylesheet with its module owner (data-plugin) when creating or reusing it.
• Prevent the client module loader from claiming this stylesheet for another plugin and removing it during reload.
• Cover ownership correction and stylesheet recreation in the lifecycle test.

Verification

• npx vitest run test/workbench-client.test.mjs (136 passed)
• npm run typecheck
• npm run build
• git diff --check origin/main codex/fix-workbench-host-style-ownership

The change is scoped to the Desktop host stylesheet; individual workbench style handling is covered separately.