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/components/ui/ResourceBar.tsx

@@ -33,7 +33,7 @@ export const ResourceBar = ({
   isSyncing,
   onForceSync,
 }: ResourceBarProps): React.JSX.Element => {
-  const { formatNumber } = useGame();
+  const { formatNumber, syncError } = useGame();
   const anyFull = Object.values(resources).some((v) => v >= RESOURCE_CAP);
   return (
   <>
@@ -68,11 +68,15 @@ export const ResourceBar = ({
       </div>
     )}
     <div className="profile-buttons">
-      {lastSavedAt !== null && (
+      {syncError !== null ? (
+        <span className="save-status save-error" title={syncError}>
+          ❌ Save failed
+        </span>
+      ) : lastSavedAt !== null ? (
         <span className="save-status" title={new Date(lastSavedAt).toLocaleString()}>
           ☁️ {formatRelativeTime(lastSavedAt)}
         </span>
-      )}
+      ) : null}
       <button
         className="force-save-button"
         disabled={isSyncing}