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/pages/CombatPage.tsx

@@ -1,5 +1,6 @@
 import { useState, useCallback } from 'react';
 import { useQuery, useMutation, useQueryClient } from '@tanstack/react-query';
+import toast from 'react-hot-toast';
 import { combatApi, characterApi } from '../api/endpoints';
 import type { Monster, CombatResult, MultiCombatResult, CombatLog } from '../api/types';
 import { Swords, Trophy, Skull, Clock, Zap, Heart, Lock } from 'lucide-react';
@@ -196,7 +197,7 @@ export function CombatPage() {
       qc.invalidateQueries({ queryKey: ['materialsInventory'] });
       startCooldown();
     },
-    onError: () => startCooldown(),
+    onError: (err: Error) => { toast.error(err.message); startCooldown(); },
   });
 
   if (isLoading) return <div style={{ padding: '2rem', color: '#6b7a99' }}>Chargement des monstres…</div>;