Implement flash alerts vs toasts split with SolidusAdmin::Layout::Flashes components

Implement flash alerts vs toasts split with SolidusAdmin::Layout::Flashes components: a task in bird-critic-1.5-full-300 (Harbor dataset). The Solidus admin currently renders all flash messages in the same way. This task separates flash handling into two distinct components…

The task

The Solidus admin currently renders all flash messages in the same way. This task separates flash handling into two distinct components: `SolidusAdmin::Layout::Flashes::Alerts::Component` for structured alert banners (keyed on `flash[:alert]`) and `SolidusAdmin::Layout::Flashes::Toasts::Component` for transient toast…

Part of kochsnow/bird-critic-1.5-full-300.