Add admin UI for custom providers

[beubax]

Summary

Allow admins to create, edit, view, and remove custom auth providers from the Authsome dashboard UI.

Motivation

Custom providers are already part of the system model, but operators should not need to use lower-level storage or CLI flows to manage provider definitions. Admins need a safe UI path for adding organization-specific OAuth, device-code, DCR/PKCE, and API-key provider definitions.

Scope

• Add an admin-only dashboard view for custom providers.
• Support listing existing custom providers.
• Support creating and editing provider definitions with validation.
• Support deleting custom providers, with clear handling when credentials or active connections reference the provider.
• Reuse the existing server-side provider definition repository and provider resolution behavior.
• Keep bundled providers read-only in the UI; custom providers may override bundled names only if the current server policy allows it.
• Ensure non-admin principals cannot access these UI actions or backing endpoints.

Acceptance Criteria

• Admins can add a custom provider through the UI and then use it in normal provider flows.
• Admins can edit and delete custom providers through the UI.
• Non-admin users cannot view or mutate the custom provider management UI or APIs.
• Validation errors are surfaced clearly before invalid provider definitions are saved.
• Existing bundled provider behavior remains unchanged.
• Audit events are emitted for create/update/delete operations.

Notes

• Server remains the business-logic owner; UI should call server APIs rather than writing provider state directly.
• Provider definitions should continue to live in the server Store through the existing repository layer.