perf(chat): debounce persisted writes and re-render only the streaming message

- the Zustand persist middleware serialized every saved session and wrote to AsyncStorage on each streamed token (appendToMessage -> set); writes are now debounced (400 ms) and flushed when the app leaves the foreground
- every token re-rendered every MessageBubble, re-parsing the Markdown of the whole conversation; rows are memoized so only the changed message renders (covered by MessageList.test.tsx)

store/useAppStore.ts

@@ -1,7 +1,7 @@
-import AsyncStorage from '@react-native-async-storage/async-storage';
 import { create } from 'zustand';
 import { createJSONStorage, persist } from 'zustand/middleware';
 import { DEFAULT_SERVER_CONFIG, LMModel, ServerConfig } from '../services/types';
+import { appStorage } from './debouncedStorage';
 
 export interface ChatMessage {
     id: string;
@@ -164,7 +164,7 @@ export const useAppStore = create<AppState>()(
         }),
         {
             name: 'localaiapp-storage',
-            storage: createJSONStorage(() => AsyncStorage),
+            storage: createJSONStorage(() => appStorage),
             partialize: (state) => ({
                 theme: state.theme,
                 serverConfig: state.serverConfig,