What this PR does

Before this PR:

• Gateway startup failures showed limited diagnostics (only stderr, no stdout)
• OpenClawPage had infinite re-render loops caused by setInterval + dispatch in useEffect deps
• Process exit with code 0 was not properly diagnosed when the daemonized child failed

After this PR:

• Capture both stdout and stderr from the gateway process for better failure diagnostics
• Replace setInterval polling with SWR for stable, dedup'd status/health polling in the UI
• Improve error messages for gateway startup timeout (combine health, stderr, stdout diagnostics)
• Handle edge case where gateway exits with code 0 but never becomes healthy

Why we need it and why it was done in this way

The infinite re-render loop in the renderer was the root cause of false timeout errors during gateway health checks. Switching to SWR provides stable polling without the pitfall of setInterval + dispatch in useEffect deps.

The diagnostics improvements (stdout capture, combined error output) are independent quality-of-life improvements that help debug gateway startup failures.

Breaking changes

None.

Checklist

• PR: The PR description is expressive enough and will help future contributors
• Code: Write code that humans can understand and Keep it simple
• Self-review: I have reviewed my own code before requesting review from others

Release note

NONE