fix: sync currentModel from SaaS available models on login

Root cause: conversationStore hardcoded 'glm-4-flash' as default model,
which may not exist in SaaS admin config, causing 404 on all chat requests.

- conversationStore: default currentModel to empty string (runtime-resolved)
- saasStore: after fetching available models, auto-switch currentModel
  to first available if the stored model is not in the list
- SaaS relay getModel() already had fallback to first available model

desktop/src/store/chat/conversationStore.ts

@@ -192,7 +192,7 @@ export const useConversationStore = create<ConversationState>()(
   agents: [DEFAULT_AGENT],
   currentAgent: DEFAULT_AGENT,
   sessionKey: null,
-  currentModel: 'glm-4-flash-250414',
+  currentModel: '',  // Set dynamically: SaaS models or config.toml default
 
   newConversation: (currentMessages: ChatMessage[]) => {
     const state = get();