feat: show save errors in the UI instead of console

Add syncError state to GameContext. forceSync now catches errors
and displays them in the ResourceBar for 5 seconds, replacing the
cloud save timestamp with '❌ Save failed'. Signature mismatches
are also cleared from localStorage so the next save can proceed.
Auto-save silently self-heals bad signatures without surfacing an error.

apps/web/src/context/GameContext.tsx

@@ -43,6 +43,8 @@ interface GameContextValue {
   isSyncing: boolean;
   /** Immediately save to the server and reset the auto-save timer */
   forceSync: () => Promise<void>;
+  /** Error message from the last failed cloud save (null when no error) */
+  syncError: string | null;
   /** Offline gold earned on login */
   offlineGold: number;
   /** Dismiss the offline gold notification */
@@ -76,6 +78,8 @@ export const GameProvider = ({ children }: { children: React.ReactNode }): React
   const [newAchievements, setNewAchievements] = useState<Achievement[]>([]);
   const [lastSavedAt, setLastSavedAt] = useState<number | null>(null);
   const [isSyncing, setIsSyncing] = useState(false);
+  const [syncError, setSyncError] = useState<string | null>(null);
+  const syncErrorTimerRef = useRef<ReturnType<typeof setTimeout> | null>(null);
   const [numberFormat, setNumberFormat] = useState<NumberFormat>("suffix");
   const stateRef = useRef<GameState | null>(null);
   const lastSaveRef = useRef<number>(Date.now());
@@ -163,6 +167,12 @@ export const GameProvider = ({ children }: { children: React.ReactNode }): React
               signatureRef.current = response.signature;
               localStorage.setItem("elysium_save_signature", response.signature);
             }
+          }).catch((err: unknown) => {
+            // Silently clear a bad signature so the next auto-save can proceed
+            if (err instanceof Error && err.message.includes("signature mismatch")) {
+              signatureRef.current = null;
+              localStorage.removeItem("elysium_save_signature");
+            }
           });
         }
       }
@@ -179,6 +189,17 @@ export const GameProvider = ({ children }: { children: React.ReactNode }): React
   // eslint-disable-next-line react-hooks/exhaustive-deps -- only run when state becomes available
   }, [state !== null]);
 
+  const showSyncError = useCallback((message: string) => {
+    setSyncError(message);
+    if (syncErrorTimerRef.current) clearTimeout(syncErrorTimerRef.current);
+    syncErrorTimerRef.current = setTimeout(() => { setSyncError(null); }, 5000);
+  }, []);
+
+  const clearBadSignature = useCallback(() => {
+    signatureRef.current = null;
+    localStorage.removeItem("elysium_save_signature");
+  }, []);
+
   const forceSync = useCallback(async () => {
     if (!stateRef.current || isSyncingRef.current) return;
     isSyncingRef.current = true;
@@ -188,17 +209,22 @@ export const GameProvider = ({ children }: { children: React.ReactNode }): React
         state: stateRef.current,
         signature: signatureRef.current ?? undefined,
       });
+      setSyncError(null);
       setLastSavedAt(response.savedAt);
       lastSaveRef.current = Date.now();
       if (response.signature) {
         signatureRef.current = response.signature;
         localStorage.setItem("elysium_save_signature", response.signature);
       }
+    } catch (err) {
+      const message = err instanceof Error ? err.message : "Save failed";
+      showSyncError(message);
+      if (message.includes("signature mismatch")) clearBadSignature();
     } finally {
       isSyncingRef.current = false;
       setIsSyncing(false);
     }
-  }, []);
+  }, [showSyncError, clearBadSignature]);
 
   const handleClick = useCallback(() => {
     setState((prev) => {
@@ -469,6 +495,7 @@ export const GameProvider = ({ children }: { children: React.ReactNode }): React
         lastSavedAt,
         isSyncing,
         forceSync,
+        syncError,
         offlineGold,
         dismissOfflineGold,
         battleResult,