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/connectionStore.ts

@@ -546,6 +546,17 @@ export const useConnectionStore = create<ConnectionStore>((set, get) => {
               await kernelClient.connect();
 
               set({ gatewayVersion: 'saas-relay', connectionState: 'connected' });
+
+              // 同步 modelToUse 到 conversationStore（首次登录时 currentModel 可能为空）
+              try {
+                const cs = await loadConversationStore();
+                const currentInStore = cs?.useConversationStore.getState().currentModel;
+                if (!currentInStore && modelToUse) {
+                  cs?.useConversationStore.getState().setCurrentModel(modelToUse);
+                  log.info(`Synced currentModel after SaaS relay connect: ${modelToUse}`);
+                }
+              } catch { /* non-critical */ }
+
               log.debug('Connected via SaaS relay (kernel backend):', {
                 model: modelToUse,
                 baseUrl: `${session.saasUrl}/api/v1/relay`,
@@ -578,6 +589,16 @@ export const useConnectionStore = create<ConnectionStore>((set, get) => {
               initializeStores();
 
               log.debug('Connected to SaaS relay (browser mode)', { relayModel: fallbackModelId });
+
+              // 同步 currentModel 到 conversationStore（浏览器路径）
+              try {
+                const cs = await loadConversationStore();
+                const currentInStore = cs?.useConversationStore.getState().currentModel;
+                if (!currentInStore && fallbackModelId) {
+                  cs?.useConversationStore.getState().setCurrentModel(fallbackModelId);
+                  log.info(`Synced currentModel after browser SaaS relay connect: ${fallbackModelId}`);
+                }
+              } catch { /* non-critical */ }
             }
             return;
           }