fix(studio): render root agent as canvas container

[FirstayZheng]

Summary

• Keep the root Agent visible as the canvas container so sub Agents are added inside the main Agent instead of replacing it.
• Move boundary insertion affordances into group containers and keep parallel groups to one bottom add action.
• Rebuild bundled WebUI assets to remove stale conflict markers that caused the custom Agent page to white-screen.

Verification

• git diff --check
• python3 -m pre_commit run --all-files
• npm test
• npm run build
• Browser smoke test: login locally, open Add Agent -> Quick Create -> Custom, verify the build canvas renders without the previous Unexpected token << error.