perf(stores): defer safeStorage writes off the interaction path (#1941)

* perf(stores): defer safeStorage writes off the interaction path

Session switches funnel every persisted store slice through safeStorage.setItem,
and doing those large JSON.stringify writes synchronously blocked the main
thread for over a second. Add a write-behind buffer that:

- Defers each setItem/removeItem to a later task via setTimeout(0) so the
  click-to-paint path is not blocked.
- Coalesces repeated writes to the same key into a single backing flush.
- Serves pending values from memory so read-after-write stays consistent
  within the deferral window.
- Flushes synchronously on pagehide/beforeunload/visibilitychange/freeze so
  deferred state survives tab close, reload, and the mobile freeze lifecycle.

Adds a test covering write deferral, coalescing, and pending read serving.

* fix(stores): defer persisted JSON serialization

* fix(stores): defer direct safeStorage writes

---------

Co-authored-by: Bohdan Triapitsyn <artmore@protonmail.com>

packages/ui/src/stores/useConfigStore.ts

@@ -1,11 +1,11 @@
 import { create } from "zustand";
 import type { StoreApi, UseBoundStore } from "zustand";
-import { devtools, persist, createJSONStorage } from "zustand/middleware";
+import { devtools, persist } from "zustand/middleware";
 import type { Provider, Agent, Config } from "@opencode-ai/sdk/v2";
 import { opencodeClient } from "@/lib/opencode/client";
 import { scopeMatches, subscribeToConfigChanges } from "@/lib/configSync";
 import type { ModelMetadata } from "@/types";
-import { getSafeStorage } from "./utils/safeStorage";
+import { createDeferredSafeJSONStorage } from "./utils/safeStorage";
 import { filterVisibleAgents } from "./useAgentsStore";
 import { isPrimaryMode } from "@/components/chat/mobileControlsUtils";
 import { useSessionUIStore } from "@/sync/session-ui-store";
@@ -3283,7 +3283,7 @@ export const useConfigStore = create<ConfigStore>()(
             }),
             {
                 name: "config-store",
-                storage: createJSONStorage(() => getSafeStorage()),
+                storage: createDeferredSafeJSONStorage(),
                 merge: (persistedState, currentState) =>
                     hydrateActiveDirectorySnapshot({
                         ...currentState,