fix: reject invalid dashboard ports

[hiSandog]

Summary

• Parse dashboard start --port through a small helper instead of silently falling back to 4848
• Report missing, invalid, or out-of-range port values as CLI errors
• Add unit coverage for default, valid, missing, and invalid dashboard port arguments

Validation

• git diff --check

I could not run cargo fmt or cargo test locally because this environment does not have cargo/rustc on PATH.

[vercel]

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