fix(desktop): 替换 require() 为 ES import — 修复生产构建崩溃

- connectionStore: 2 处 require() → loadConversationStore() 异步预加载 + 闭包引用
- saasStore: 1 处 require() → await import()（logout 是 async）
- llm-service: 1 处 require() → 顶层 import（无循环依赖）
- streamStore: 移除重复动态导入，统一使用顶层 useConnectionStore
- tsc --noEmit 0 errors

desktop/src/store/saasStore.ts

@@ -434,7 +434,7 @@ export const useSaaSStore = create<SaaSStore>((set, get) => {
 
       // Clear currentModel so next connection uses fresh model resolution
       try {
-        const { useConversationStore } = require('./chat/conversationStore');
+        const { useConversationStore } = await import('./chat/conversationStore');
         useConversationStore.getState().setCurrentModel('');
       } catch { /* non-critical */ }