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/constants/models.ts

@@ -51,11 +51,11 @@ export const PROVIDER_DEFAULTS = {
   },
   zhipu: {
     baseUrl: 'https://open.bigmodel.cn/api/paas/v4',
-    defaultModel: 'glm-4-flash',
+    defaultModel: 'glm-4-flash-250414',
   },
   zhipu_coding: {
     baseUrl: 'https://open.bigmodel.cn/api/coding/paas/v4',
-    defaultModel: 'glm-4-flash',
+    defaultModel: 'glm-4-flash-250414',
   },
   kimi: {
     baseUrl: 'https://api.moonshot.cn/v1',