fix: guard claimedMilestones undefined on v1 saves

Saves loaded before migrateSave runs (or stale localStorage) can have
claimedMilestones undefined — guard with ?? [] in all accessors.

Frontend/src/core/economy.ts

@@ -186,8 +186,9 @@ export function computePrestigeDna(lifetimeTadpoles: number, prestigeCount: numb
 
 // Milestones disponibles mais pas encore réclamés
 export function getClaimableMilestones(state: GameState): PrestigeMilestone[] {
+  const claimed = state.claimedMilestones ?? [];
   return PRESTIGE_MILESTONES.filter(
-    (m) => state.prestigeCount >= m.threshold && !state.claimedMilestones.includes(m.id)
+    (m) => state.prestigeCount >= m.threshold && !claimed.includes(m.id)
   );
 }
 
@@ -201,11 +202,12 @@ export function claimMilestone(state: GameState, milestoneId: string): GameState
   const milestone = PRESTIGE_MILESTONES.find((m) => m.id === milestoneId);
   if (!milestone) return null;
   if (state.prestigeCount < milestone.threshold) return null;
-  if (state.claimedMilestones.includes(milestoneId)) return null;
+  const claimed = state.claimedMilestones ?? [];
+  if (claimed.includes(milestoneId)) return null;
 
   let newState = {
     ...state,
-    claimedMilestones: [...state.claimedMilestones, milestoneId],
+    claimedMilestones: [...claimed, milestoneId],
   };
 
   // Appliquer la récompense
@@ -224,13 +226,13 @@ export function claimMilestone(state: GameState, milestoneId: string): GameState
 
 // Bonus gameplay cumulés depuis les milestones réclamés
 export function getMilestoneStartNid(state: GameState): number {
-  const claimed = state.claimedMilestones;
+  const claimed = state.claimedMilestones ?? [];
   if (claimed.includes("milestone_5")) return 1; // 1 Nid gratuit
   return 0;
 }
 
 export function getMilestoneOfflineBonus(state: GameState): number {
-  const claimed = state.claimedMilestones;
+  const claimed = state.claimedMilestones ?? [];
   if (claimed.includes("milestone_15")) return 0.05; // +5% offline cap
   return 0;
 }