fix(auth): 修复重启后无法对话 — restoreSession 优先验证 SaaS token

根因: 心跳降级将 'tauri' 持久化到 localStorage，重启后盲信该值。
修复: token refresh 成功时强制恢复 'saas' 模式；connectionMode 携带时间戳。

desktop/src/store/saasStore.ts

@@ -818,8 +818,11 @@ export const useSaaSStore = create<SaaSStore>((set, get) => {
         account,
         saasUrl: restored.saasUrl,
         authToken: newToken,
-        connectionMode: loadConnectionMode() === 'saas' ? 'saas' : 'tauri',
+        // If token refresh succeeded, always restore to 'saas' mode
+        // regardless of what was persisted (heartbeat may have degraded to 'tauri')
+        connectionMode: 'saas',
       });
+      saveConnectionMode('saas');
       get().fetchAvailableModels().catch(() => {});
       get().fetchAvailableTemplates().catch(() => {});
       get().fetchAssignedTemplate().catch(() => {});