[BUG] Linux: login dead-ends at "Finish sign-in in the Claude app"

### Preflight Checklist

- [x] I have searched [existing issues](https://github.com/anthropics/claude-code/issues?q=is%3Aissue%20state%3Aopen%20label%3Abug) and this hasn't been reported yet
- [x] This is a single bug report (please file separate reports for different bugs)
- [x] I am using the latest version of Claude Code

### What's Wrong?

when trying to login (check steps to reproduce)
The page triggers a claude:// deep link. GNOME has no registered handler,
so it opens "Open With… / No Apps Available — No apps installed that can
open '..._nonce=...'". Sign-in never completes.

Notes
- Reproduces with both Google auth and the email verification code path,
so it is not provider-specific — the failure is the app handoff.
- The desktop app ships for macOS/Windows only, so on Linux there is
nothing for the deep link to open. The web/CLI flow appears to be
reusing the desktop handoff.

### What Should Happen?

Sign-in completes on Linux — e.g. redirect to a localhost callback, or
show a code to paste back into the terminal.

### Error Messages/Logs

```shell

```

### Steps to Reproduce

Environment
- OS: Linux (GNOME), kernel 7.0.0-31-generic
- Browser: Firefox
- Entry point:

Steps to reproduce
1. Start sign-in on Linux.
2. Authenticate with Google (also reproduces with email code).
3. Browser lands on claude.ai/login/popup-google-auth showing
"Finish sign-in in the Claude app" with an "Open Claude" button.
4. Click "Open Claude".

### Claude Model

Opus

### Is this a regression?

Yes, this worked in a previous version

### Last Working Version

_No response_

### Claude Code Version

2.1.273

### Platform

Anthropic API

### Operating System

Ubuntu/Debian Linux

### Terminal/Shell

Other

### Additional Information

_No response_

Reproduce the flow from through claude.ai/login/popup-google-auth on Linux with GNOME and Firefox, then inspect how the browser's claude:// deep link is handed back to the CLI. Done means Google and email-code sign-in complete on Linux without requiring an unavailable desktop app, using a supported callback or terminal handoff.

Written by the indexing model from the issue text.