Add toast notification system for user feedback #24

Summary

The app has no feedback mechanism for async actions (deploy success/failure, copy to clipboard, import errors). This issue adds a lightweight toast notification system.

Tasks

• Install or implement a minimal toast system (consider sonner or a custom useToast hook)
• Create a <Toaster> component mounted at the root layout
• Expose a toast.success(), toast.error(), and toast.info() API
• Use toasts for: deploy success (with contract ID), deploy error, copy to clipboard, import validation error, save confirmation

Acceptance Criteria

• Success, error, and info toasts render with distinct colours/icons
• Toasts auto-dismiss after 4 seconds
• Multiple toasts stack without overlapping
• Toasts are announced to screen readers via aria-live