fix(helm): add startupProbe for hubble-ui backend

[Sébastien Larivière (slariviere)]

Description

Adds a startupProbe for the hubble-ui backend container.

The backend dials hubble-relay before it starts serving /healthz, and hubble-relay itself can take up to ~60s to become healthy on startup. Without a startupProbe, the default liveness/readiness tolerance (~30s) can kill the backend before it ever converges, causing an unrecoverable restart loop whenever hubble-relay is slow to start. This mirrors the existing startupProbe pattern already used for hubble-relay itself.

Checklist

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• I have added tests, if applicable.