Problem

The OAuth page needs a user-facing way to provide an explicit proxy for a single login attempt. Users also need a convenient way to copy the configured global proxy into that one-time OAuth proxy field when they want the generated auth file to persist it.

Proposal

Add OAuth proxy controls to the Web UI:

• A page-level OAuth proxy URL input shared by all OAuth providers.
• A button that fills the OAuth proxy input with the current global proxy.
• No localStorage persistence for the OAuth proxy input.

Expected behavior

• Empty OAuth proxy input continues to let the backend inherit the global proxy without writing proxy_url into auth files.
• Explicit OAuth proxy input is sent as proxy_url and becomes persisted by the backend.
• The global proxy fill action only copies the current global proxy into the input; it does not auto-submit or permanently store the value in the browser.