fix(6): add missing Toast component and fix backend startup

- Create Toast component for success/error messages
- Fix uvicorn invocation: use backend.main:app from project root
- Ensures relative imports work correctly in backend modules

frontend/components/Toast.tsx

@@ -0,0 +1,34 @@
+'use client';
+
+import { useEffect } from 'react';
+
+export interface ToastProps {
+  type: 'success' | 'error' | 'warning' | 'info';
+  message: string;
+  onClose: () => void;
+  duration?: number;
+}
+
+export function Toast({ type, message, onClose, duration = 3000 }: ToastProps) {
+  useEffect(() => {
+    const timer = setTimeout(onClose, duration);
+    return () => clearTimeout(timer);
+  }, [onClose, duration]);
+
+  const bgColor = {
+    success: 'bg-green-500',
+    error: 'bg-red-500',
+    warning: 'bg-yellow-500',
+    info: 'bg-blue-500',
+  }[type];
+
+  return (
+    <div
+      className={`fixed bottom-4 right-4 ${bgColor} text-white px-4 py-2 rounded shadow-lg z-50 animate-fade-in`}
+      role="alert"
+      aria-live="polite"
+    >
+      {message}
+    </div>
+  );
+}