feat: Sprint 3 — Prestige Loop endless

- Migration saves: saveVersion pattern + migrateSave lazy (v1→v2)
- Formule ADN rebalancée: log10 + clamp min 1 + cap bonus ×4
- Prestige Experience: modal fullscreen, preview ADN, stats run, best run
- Arbre V2: 25 nœuds, 3 capstones, post-capstones repeatables (scaling par tranche)
- Convergence évolutif Alpha→Omega (tier system)
- Reset arbre: 1 gratuit/prestige, payant linéaire au-delà
- Milestones prestige: 8 paliers (1→100), cosmétiques exclusifs, bonus gameplay
- balance.ts: constantes centralisées pour playtest
- 136 tests green, 0 regression

Frontend/src/hooks/useSaveSync.ts

@@ -5,6 +5,7 @@ import { useEffect, useRef, useCallback, useState } from "react";
 import { useAuth } from "../context/AuthContext";
 import { useGameStore } from "../store/useGameStore";
 import type { GameState } from "../core/economy";
+import { migrateSave } from "../core/migrateSave";
 
 const SAVE_INTERVAL_MS = 30_000; // 30 seconds
 const BACKEND_URL = import.meta.env.VITE_BACKEND_URL || "http://localhost:3310";
@@ -51,9 +52,10 @@ export function useSaveSync({ getGameState, onLoad, playTimeSeconds }: SaveSyncO
 
     apiRequest("/save").then((data) => {
       if (data?.gameState) {
-        onLoad(data.gameState);
+        const migrated = migrateSave(data.gameState);
+        onLoad(migrated);
         lastSaveRef.current = data.lastSave;
-        console.info("[SaveSync] Loaded save from server — server is authority");
+        console.info("[SaveSync] Loaded save from server — server is authority (v%d)", migrated.saveVersion);
       } else {
         console.info("[SaveSync] No server save found — starting fresh");
       }
@@ -99,7 +101,8 @@ export function useSaveSync({ getGameState, onLoad, playTimeSeconds }: SaveSyncO
       setTimeout(() => apiRequest("/save").then((data) => {
         if (data?.gameState && data.lastSave) {
           if (!lastSaveRef.current || new Date(data.lastSave) > new Date(lastSaveRef.current)) {
-            onLoad(data.gameState);
+            const migrated = migrateSave(data.gameState);
+            onLoad(migrated);
             lastSaveRef.current = data.lastSave;
             console.info("[SaveSync] Reloaded from server on focus");
           }