Desktop: No Custom Headers field for custom OpenAI-compatible providers

[dianed-square]

Describe the bug

Desktop UI does not provide a way to configure custom headers for OpenAI-compatible custom providers. CLI supports this feature, but Desktop users have no equivalent option.

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To Reproduce

1. Open goose Desktop > Settings > Models > Configure providers
2. Click Add Custom Provider
3. Select OpenAI Compatible as the Provider Type
4. Observe that there is no field for custom headers
5. Compare with CLI: run goose configure > Custom Providers > Add A Custom Provider > OpenAI Compatible where you're prompted to add custom headers

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Expected behavior Desktop should provide a way to configure/view/edit custom headers for OpenAI-compatible custom providers, matching CLI functionality.

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Screenshots N/A

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Please provide the following information

• OS & Arch: macOS
• Interface: UI (goose Desktop)
• Version: v1.16.0
• Extensions enabled: N/A
• Provider & Model: Custom OpenAI-compatible provider

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Additional context PR #5017 added backend support for custom headers, including:

• UpdateCustomProviderRequest includes headers field
• openapi.json and types.gen.ts have the schema
• CLI flow works correctly

Fix: Add a headers input field to CustomProviderForm.tsx that captures and submits headers using the existing backend support.

[Lymah123]

@dianed-square , can I work on this?

[Abhijay007]

@dianed-square , can I work on this?

sure @Lymah123 go ahead