fix: improve error handling when OAuth callback port is in use

[kiranjd]

Problem

1. Press Sign in in app
2. Redirects to GitHub
3. Login succeeds
4. Triggers redirect to localhost:53682/callback
5. "Site can't be reached" - callback server never started because port was held by stale connection

Fix

• Check if port is available before binding
• Throw clear error: "Port 53682 is already in use. Try again in a few seconds, or specify a different port with --loopback-port."

[kiranjd]

Screenshot of the error

[steipete]

Thanks Kiran!

[steipete]

Landed via temp rebase onto main.\n\n- Gate: pnpm lint && pnpm build && pnpm test\n- Land commit: d39a19f\n- Merge commit: a603e59\n\nFollow-up cherry-pick onto main to include test + messaging adjustments.\n\nThanks @kiranjd!