fix(desktop): 模型自动路由 — 首次登录自动选择可用模型

- saasStore: fetchAvailableModels 处理 currentModel 为空的情况，自动选择第一个可用模型
- connectionStore: SaaS relay 连接成功后同步 currentModel 到 conversationStore
- 同时覆盖 Tauri 和浏览器两条 SaaS relay 路径
- 修复首次登录用户需手动选模型的问题

desktop/src/store/saasStore.ts

@@ -488,11 +488,15 @@ export const useSaaSStore = create<SaaSStore>((set, get) => {
             const { useConversationStore } = await import('./chat/conversationStore');
             const current = useConversationStore.getState().currentModel;
             const modelIds = models.map(m => m.alias || m.id);
-            if (current && !modelIds.includes(current)) {
-              const firstModel = models[0];
-              const fallbackId = firstModel.alias || firstModel.id;
+            const firstModel = models[0];
+            const fallbackId = firstModel.alias || firstModel.id;
+            if (!current || !modelIds.includes(current)) {
               useConversationStore.getState().setCurrentModel(fallbackId);
-              log.info(`Synced currentModel: ${current} not available, switched to ${fallbackId}`);
+              if (current) {
+                log.info(`Synced currentModel: ${current} not available, switched to ${fallbackId}`);
+              } else {
+                log.info(`Auto-selected first available model: ${fallbackId}`);
+              }
             }
           } catch (syncErr) {
             log.warn('Failed to sync currentModel after fetching models:', syncErr);