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/styles.css

@@ -1201,6 +1201,11 @@ body {
   white-space: nowrap;
 }
 
+.save-error {
+  color: #ef4444;
+  font-weight: 600;
+}
+
 .force-save-button {
   align-items: center;
   background: rgba(255, 255, 255, 0.06);