feat(oauth): implement redirect flow for device authorization

[nycomp]

Goal

Complete the OAuth 2.0 Device Authorization Flow (RFC 8628) by implementing the redirect flow after user authorization.

Current State

• Device code endpoint (/auth/v1/oauth/device_authorize) works ✅
• Token endpoint (/auth/v1/oauth/token) with polling works ✅
• Verification page exists at /auth/v1/oauth/device ✅
• User can submit user code and authorize ✅

Missing

After the user authorizes the device code, the verification page needs to:

1. Show success message - "Authorization complete! You can close this page and return to the CLI."
2. Auto-close (optional) - Use JavaScript to close the window after showing success
3. Handle errors gracefully - Show clear messages for expired/denied codes

Reference

• RFC 8628 Section 3.3: https://datatracker.ietf.org/doc/html/rfc8628#section-3.3
• Feature branch: feature/oauth-device-authorization-flow
• Related CLI PR: campus-cli device auth implementation

Acceptance Criteria

• Verification page shows clear success message after authorization
• Error states (expired, denied) show user-friendly messages
• Page works without JavaScript (progressive enhancement)
• Optional: Auto-close window after successful authorization