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.

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

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

Screenshots
N/A

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

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.