Open default world instead of home screen on delete scene

[twells46]

Fix #448 by routing to /scene/jbcSandbox instead of /.

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Note

Redirects users to the default simulator scene after deleting a scene and centralizes the default scene path.

• Add DEFAULT_SCENE constant (/scene/jbcSandbox) and use it in Dashboard.tsx and Root.tsx
• Change delete flow in Root.tsx: close modal, navigate to DEFAULT_SCENE, then reload
• Update Db.ts to support request keepalive and set it on DELETE requests to ensure completion during navigation

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