Add flush groups cache button to admin settings

[Copilot]

Group list changes (e.g. after updating group tags) aren't reflected in the admin because coreapi_groups and coreapi_all_groups transients are cached for 24 hours. The on_site_groups_option_update hook only fires when the options themselves change, not when upstream group data changes.

Changes

• New "Flush Groups Cache" button on the BCC Login settings page, rendered between the settings form and the existing Maintenance section
• Only visible when groups are enabled — reuses the same $use_groups_settings condition that gates the Groups settings section
• Handler deletes coreapi_groups and coreapi_all_groups transients with nonce verification and manage_options capability check
• Stores $_use_groups_settings as an instance property on BCC_Login_Settings_Provider so the render method can access it