fix: prevent login from hanging in CLI environments

[Alan-221b]

Fixes #1

Summary

• Run browser.OpenURL() in a goroutine to avoid blocking
• Add helpful message for manual URL copying when browser doesn't open

Problem

When running keyway login in certain CLI environments (VS Code integrated terminal, SSH sessions, headless servers), the login flow hangs after displaying the verification URL. The CLI never starts polling for OAuth completion.

Root Cause

The browser.OpenURL() call blocks in these environments, waiting for the browser process to complete, which prevents the polling loop from starting.

Before

│ Code: ABCD-1234
│ Open: https://api.keyway.sh/v1/auth/device/verify?user_code=ABCD-1234
[hangs indefinitely - no "Waiting for authorization..." spinner]

After

│ Code: ABCD-1234
│ Open: https://api.keyway.sh/v1/auth/device/verify?user_code=ABCD-1234
│ If the browser doesn't open, copy the URL above and paste it in your browser.
[polling starts immediately]

Test Plan

• Test keyway login in VS Code integrated terminal
• Test keyway login in standard terminal (macOS/Linux)
• Test keyway login via SSH session
• Verify OAuth flow completes successfully after browser authorization

[NicolasRitouet]

@Alan-221b thanks a lot for your contribution.
your changes will be released today 🎉