[client] Support BROWSER env for login

[lixmal]

Describe your changes

Issue ticket number and link

Fixes #2172

Stack

Checklist

• Is it a bug fix
• Is a typo/documentation fix
• Is a feature enhancement
• It is a refactor
• Created tests that fail without the change (if possible)

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Documentation

Select exactly one:

• I added/updated documentation for this change
• Documentation is not needed for this change (explain why)

Docs PR URL (required if "docs added" is checked)

Paste the PR link from https://github.com/netbirdio/docs here:

https://github.com/netbirdio/docs/pull/__

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[Copilot]

Pull Request Overview

This PR adds support for the BROWSER environment variable to allow users to specify a custom browser for the NetBird client login process. This addresses issue #2172 by providing more control over which browser is used when opening authentication URLs.

Key changes:

• Added BROWSER environment variable support in both UI and CLI login flows
• Replaced direct usage of the open-golang library with custom browser handling logic
• Maintained fallback behavior to existing platform-specific browser opening methods

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
client/ui/client_ui.go	Removed open-golang import and updated SSO login to use custom openURL function with BROWSER env support
client/cmd/login.go	Added openBrowser function that respects BROWSER environment variable before falling back to open.Run()

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