Desktop: honor backend host and port flags

[NicoJoerger]

Summary

• forward compatible Desktop launch flags for backend host and port
• use the configured local endpoint for Desktop health, auth, ownership, and preflight flows
• preserve wildcard binds through loopback and reject non-local hostnames / port 0
• update the WebView endpoint handoff for configured hosts and IPv6

Validation

• cargo test (372 passed)
• npm run build
• cargo build --release

Related to #8578 and #8790.