feat: Add Toaster notification system using @radix-ui/react-toast

[derekliutao-cmd]

Summary

Closes #11

Implements a full toast notification system as specified in the issue.

Changes

src/hooks/use-toast.ts (new file) — Zustand-based toast store:

• toast.success(), toast.error(), toast.info() methods
• Auto-dismiss after 5 seconds
• dismissToast(id) for manual dismissal

src/components/ui/toaster.tsx — Full implementation replacing the stub:

• Wired to @radix-ui/react-toast (already in package.json)
• Renders active toasts from useToast() hook
• Bottom-right corner positioning via ToastPrimitives.Viewport
• Success (green) / Error (red) / Info (blue) variants with icons
• Error toasts have role="alert" for accessibility
• Slide-in/fade-out animations
• Close button on each toast

Acceptance Criteria Met

• Toaster component functional using @radix-ui/react-toast
• useToast() hook with .success(), .error(), .info() methods
• Toasts auto-dismiss after 5 seconds
• Toasts appear in bottom-right corner
• Accessible (role="alert" for errors)

Uses existing project dependencies only (zustand, @radix-ui/react-toast, lucide-react).