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/package.json

@@ -14,6 +14,7 @@
     "lucide-react": "^0.577.0",
     "react": "^19.2.4",
     "react-dom": "^19.2.4",
+    "react-hot-toast": "^2.6.0",
     "react-router-dom": "^7.13.1"
   },
   "devDependencies": {