feat: add session export with full and compressed modes (#507)

Add export functionality that allows users to download session data
as JSON or plain text, with optional LLM-based context compression
for long conversations. Includes UI controls in chat panel, session
list, and history view, plus i18n strings for all 8 locales.

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

packages/client/src/i18n/locales/zh.ts

@@ -174,6 +174,12 @@ export default {
     monitorRoleUser: '用户',
     monitorRoleAssistant: '助手',
     copySessionId: '复制会话 ID',
+    export: '导出',
+    exportFull: '全量导出 (JSON)',
+    exportCompressed: '压缩导出 (TXT)',
+    exportCompressing: '正在压缩上下文，请稍候...',
+    exportSuccess: '会话已导出',
+    exportFailed: '导出失败',
     renamed: '已重命名',
     renameFailed: '重命名失败',
     renameSession: '重命名会话',