Make new proxies to have an initial self-signed certificate

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No files or tests are mentioned in the issue body. Start by locating the new-proxy creation path and the certificate provisioning logic in this JavaScript codebase, including where Let's Encrypt flows are triggered. Add a default self-signed certificate step for fresh proxies in that flow so HTTPS is configured immediately. Done means a new proxy gets an initial self-signed cert created and attached, while existing certificate behavior remains intact.

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