feat: toast system — feedback visuel global (react-hot-toast)

- Toaster dark theme (bottom-right, 3s/4s)
- Combat: erreur cooldown/endurance en toast
- Craft: toast start + collect + erreurs
- Forge: toast succès/échec + erreurs
- Shop: toast achat + erreurs
- Inventaire: toast vente + erreurs
- Fix forge costs frontend (200/400/700)

frontend/src/App.tsx

@@ -1,5 +1,6 @@
 import { BrowserRouter, Routes, Route, Navigate } from 'react-router-dom';
 import { QueryClient, QueryClientProvider } from '@tanstack/react-query';
+import { Toaster } from 'react-hot-toast';
 import { AuthProvider, useAuth } from './context/AuthContext';
 import { Layout } from './components/Layout';
 import { LoginPage }     from './pages/LoginPage';
@@ -54,6 +55,15 @@ export default function App() {
           <AppRoutes />
         </BrowserRouter>
       </AuthProvider>
+      <Toaster
+        position="bottom-right"
+        toastOptions={{
+          duration: 3000,
+          style: { background: '#1e2535', color: '#dce4f0', border: '1px solid #2a3448', fontSize: 13 },
+          success: { iconTheme: { primary: '#3ddc84', secondary: '#1e2535' } },
+          error: { iconTheme: { primary: '#e84040', secondary: '#1e2535' }, duration: 4000 },
+        }}
+      />
     </QueryClientProvider>
   );
 }