🔔 UX: Implement unified Toast Notification system

[shard-c6]

Description

Currently, feedback for user actions (success/error) might be inconsistent or rely on generic browser alerts. We should implement a unified, non-intrusive toast notification system.

Tasks

• Integrate a toast library (e.g., react-hot-toast or sonner) or build a custom context.
• Replace any generic alerts or silent failures with descriptive error toasts.
• Add success toasts for key actions (e.g., 'Plant added to collection', 'Settings saved').
• Ensure toasts are accessible (screen reader announcements).