fix: handle non-ok HTTP responses in connection store checkHealth

checkHealth previously ignored non-ok responses (e.g. 503), silently
doing nothing when the server reported unhealthy. Add else branch to
transition connected → disconnected on non-ok status. Add tests for
connected → disconnected on fetch failure and offline → reconnected
on health check success.

apps/web/src/stores/connection-store.ts

@@ -51,6 +51,10 @@ export const useConnectionStore = create<ConnectionState>((set, get) => ({
         } else {
           set({ lastHealthCheck: Date.now() });
         }
+      } else {
+        if (get().status === "connected") {
+          get().setDisconnected();
+        }
       }
     } catch {
       if (get().status === "connected") {