feat(auth): add per-auth use_global_proxy configuration

[HustCoderHu]

#304

• Add use_global_proxy field to Auth struct and auth files
• Prompt users for proxy preference during login flows
• Use provider-specific defaults (iFlow/Qwen: false, others: true)
• Maintain backward compatibility by defaulting to true for existing files
• Implement GlobalProxySetter interface for token storage
• Update proxy selection logic to respect use_global_proxy flag

This allows users to control proxy usage on a per-auth basis while maintaining backward compatibility with existing auth files.

Generated with Claude Code

[gemini-code-assist]

Summary of Changes

Hello @HustCoderHu, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces a significant enhancement to the authentication system by providing granular control over proxy usage. It allows users to configure whether a global proxy should be applied to individual authentication methods, thereby improving flexibility and user experience. The changes ensure backward compatibility for existing configurations and streamline the proxy selection process during runtime, making proxy management more intuitive and adaptable.

Highlights

• Per-Authentication Proxy Configuration: Introduced a use_global_proxy field to the Auth struct and related authentication files, allowing users to specify proxy usage on a per-authentication basis rather than globally.
• User Prompt for Proxy Preference: During login flows, users are now prompted to decide if they want to use the global proxy from config.yaml for the specific authentication being set up.
• Provider-Specific Defaults: Default use_global_proxy settings are applied based on the authentication provider; for instance, it defaults to false for iFlow and Qwen, and true for other providers.
• Backward Compatibility: Existing authentication files will default to use_global_proxy: true when loaded, ensuring that current configurations continue to function as expected without disruption.
• GlobalProxySetter Interface: A new interface GlobalProxySetter has been introduced, which allows token storage objects to dynamically update their use_global_proxy value after initial creation.
• Updated Proxy Selection Logic: The runtime proxy resolution logic has been enhanced to respect the new use_global_proxy flag, ensuring that the global proxy is only applied if explicitly enabled for a given authentication.

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[gemini-code-assist]

Code Review

This pull request introduces a per-authentication use_global_proxy setting, which is a great feature for giving users more granular control over their proxy configurations. The implementation is well-thought-out, covering backward compatibility for existing auth files and centralizing the new logic where possible. The use of the GlobalProxySetter interface is a clean approach to update token storage. My review includes a couple of suggestions for the iflow_cookie.go command to improve code clarity and reduce duplication.

[HustCoderHu]

@luispater