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/en.ts

@@ -174,6 +174,12 @@ export default {
     monitorRoleUser: 'User',
     monitorRoleAssistant: 'Assistant',
     copySessionId: 'Copy Session ID',
+    export: 'Export',
+    exportFull: 'Full Export (JSON)',
+    exportCompressed: 'Compressed Export (TXT)',
+    exportCompressing: 'Compressing context, please wait...',
+    exportSuccess: 'Session exported',
+    exportFailed: 'Export failed',
     renamed: 'Renamed',
     renameFailed: 'Rename failed',
     renameSession: 'Rename Session',