Settings: Add Auth tab for centralized handler authentication management

[saraichinwag]

Summary

Add an Auth tab to the Data Machine settings React app that shows all handlers with auth requirements, their connection status, and allows connect/disconnect from a single view.

Problem

Currently, all handler authentication is managed from the pipelines page — you have to find a step that uses a specific handler to check or manage its auth status. There's no overview of which handlers are connected, which need attention, or a single place to manage credentials.

Proposed Solution

New Auth tab in the settings React app (alongside Handler Defaults, etc.) that:

• Lists all registered handlers that have auth requirements
• Shows connection status for each (connected / disconnected / expired)
• Provides connect/disconnect actions inline
• Triggers the same OAuth flows already used on the pipelines page

Key Constraint: No Duplication

The pipelines page already has auth UI components (OAuth redirect handling, status display, connect/disconnect buttons). The settings Auth tab must share the same underlying React modules — same API calls, same OAuth redirect handling, same status display logic. Different mounting points, shared internals.

This is feasible since both the settings app and pipelines page are React and headless.

Scope

• New Auth tab component in the settings React app
• Extract shared auth modules from pipeline page components (if not already extracted)
• Handler auth status API (may already exist via handler endpoints — needs audit)
• Connect/disconnect actions per handler
• Visual status indicators (connected, disconnected, token expired, etc.)