feat: server-authoritative save — wait for server before game starts

- Store starts in ready:false — no game actions until authority loaded
- useSaveSync signals serverLoaded (success, empty, or unreachable)
- GameSync orchestrates: logged in → wait server, guest → localStorage
- Home shows loading screen until ready
- localStorage = cache only, server = authority for logged-in users

Frontend/src/components/GameSync.tsx

@@ -1,22 +1,47 @@
 // GameSync.tsx — Bridge useSaveSync ↔ Zustand store
-// Monter une seule fois dans App. Silencieux en mode invité (pas de token).
+// Serveur = autorité. Attend la save serveur avant de rendre le jeu jouable.
+// Guest mode (pas connecté) : init depuis localStorage immédiatement.
 
-import { useCallback } from "react";
+import { useCallback, useEffect, useRef } from "react";
 import { useGameStore } from "../store/useGameStore";
+import { useAuth } from "../context/AuthContext";
 import { useSaveSync } from "../hooks/useSaveSync";
 
 export function GameSync() {
   const state = useGameStore((s) => s.state);
+  const ready = useGameStore((s) => s.ready);
   const loadFromServer = useGameStore((s) => s.loadFromServer);
+  const initGuest = useGameStore((s) => s.initGuest);
   const playSeconds = useGameStore((s) => s.playSeconds);
+  const { user, loading: authLoading } = useAuth();
+  const initDone = useRef(false);
 
   const getGameState = useCallback(() => state, [state]);
 
-  useSaveSync({
+  const { serverLoaded } = useSaveSync({
     getGameState,
     onLoad: loadFromServer,
     playTimeSeconds: playSeconds,
   });
 
+  // Once auth resolves: if no user or no server save → init guest
+  useEffect(() => {
+    if (authLoading || initDone.current || ready) return;
+
+    // Not logged in → guest mode immediately
+    if (!user) {
+      initDone.current = true;
+      initGuest();
+      return;
+    }
+
+    // Logged in but server save loaded (or confirmed empty) → useSaveSync handles it
+    // If serverLoaded is true and store isn't ready yet, it means server had no save
+    if (serverLoaded && !ready) {
+      initDone.current = true;
+      initGuest(); // use localStorage as starting point, server will save it on next sync
+    }
+  }, [authLoading, user, serverLoaded, ready, initGuest]);
+
   return null;
 }