Add toast notifications for user feedback

- Created ToastContext with showToast hook
- Toast notifications appear in bottom-right, auto-dismiss after 3s
- Added success/error toasts for:
  - Saving notification settings
  - Refreshing prices
  - Updating check interval
  - Deleting products
- Replaced alert() calls with toast notifications

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/App.tsx

@@ -1,6 +1,7 @@
 import { useEffect } from 'react';
 import { BrowserRouter, Routes, Route, Navigate } from 'react-router-dom';
 import { AuthProvider, useAuth } from './context/AuthContext';
+import { ToastProvider } from './context/ToastContext';
 import Login from './pages/Login';
 import Register from './pages/Register';
 import Dashboard from './pages/Dashboard';
@@ -120,7 +121,9 @@ export default function App() {
     <ThemeInitializer>
       <BrowserRouter>
         <AuthProvider>
-          <AppRoutes />
+          <ToastProvider>
+            <AppRoutes />
+          </ToastProvider>
         </AuthProvider>
       </BrowserRouter>
     </ThemeInitializer>