fix: migrate glm-4-flash to glm-4-flash-250414 (model deprecated by Zhipu)

Zhipu AI has deprecated glm-4-flash, causing 404 errors on all chat requests.
Updated all references:
- config: glm-4-flash → glm-4-flash-250414, added glm-z1-flash
- frontend: defaultModel, conversationStore, ChatArea fallback, ModelsAPI

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',
+  currentModel: 'glm-4-flash-250414',
 
   newConversation: (currentMessages: ChatMessage[]) => {
     const state = get();