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/hooks/useSaveSync.ts

@@ -1,7 +1,7 @@
 // useSaveSync.ts — Auto-save game state to backend every 30s
-// Cookie-based auth — credentials sent automatically
+// Serveur = autorité. Cookie-based auth — credentials sent automatically.
 
-import { useEffect, useRef, useCallback } from "react";
+import { useEffect, useRef, useCallback, useState } from "react";
 import { useAuth } from "../context/AuthContext";
 import type { GameState } from "../core/economy";
 
@@ -37,18 +37,29 @@ export function useSaveSync({ getGameState, onLoad, playTimeSeconds }: SaveSyncO
   const { user } = useAuth();
   const lastSaveRef = useRef<string | null>(null);
   const loadedRef = useRef(false);
+  const [serverLoaded, setServerLoaded] = useState(false);
 
-  // Load save on mount (once)
+  // Load save from server on mount (once, only if logged in)
   useEffect(() => {
-    if (loadedRef.current || !user) return;
+    if (loadedRef.current || !user) {
+      // Not logged in → signal immediately
+      if (!user) setServerLoaded(true);
+      return;
+    }
     loadedRef.current = true;
 
     apiRequest("/save").then((data) => {
       if (data?.gameState) {
         onLoad(data.gameState);
         lastSaveRef.current = data.lastSave;
-        console.info("[SaveSync] Loaded save from server");
+        console.info("[SaveSync] Loaded save from server — server is authority");
+      } else {
+        console.info("[SaveSync] No server save found — starting fresh");
       }
+      setServerLoaded(true);
+    }).catch(() => {
+      console.warn("[SaveSync] Server unreachable — falling back to local");
+      setServerLoaded(true);
     });
   }, [onLoad, user]);
 
@@ -99,5 +110,5 @@ export function useSaveSync({ getGameState, onLoad, playTimeSeconds }: SaveSyncO
     return () => window.removeEventListener("beforeunload", handleUnload);
   }, [getGameState, playTimeSeconds, user]);
 
-  return { saveToServer, lastSave: lastSaveRef.current };
+  return { saveToServer, lastSave: lastSaveRef.current, serverLoaded };
 }