feat(conversation): enhance empty state UI and improve quick action descriptions

- Add new chat empty state image asset (chatEmpty.png)
- Update English quick action descriptions with more compelling copy for applications, knowledge base, memory conversation, and help center
- Update Chinese quick action descriptions with concise, marketing-focused messaging
- Replace conversation empty state image from generic background to dedicated chat empty illustration
- Improve user experience with clearer value propositions for each quick action feature

web/src/i18n/zh.ts

@@ -782,15 +782,15 @@ export const zh = {
       createNewMemorySummary: '创建新记忆条目',
 
       createNewApplication: '创建新应用',
-      createNewApplicationDesc: '创建新空间应用',
+      createNewApplicationDesc: '零代码拖拽3分钟创应用',
 
-      createNewKnowledge: '创建新知识',
-      createNewKnowledgeDesc: '创建新记忆条目',
+      createNewKnowledge: '创建知识库',
+      createNewKnowledgeDesc: '秒变可搜索的专属知识库',
 
       memoryConversation: '记忆对话',
-      memoryConversationDesc: '记忆对话',
+      memoryConversationDesc: '让AI越用越懂你',
       helpCenter: '帮助中心',
-      helpCenterDesc: '帮助中心',
+      helpCenterDesc: '一站式解决疑问快速上手',
       memorySummary: '查看记忆摘要',
       memorySummaryDesc: '查看记忆摘要报告',