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/de.ts

@@ -151,6 +151,12 @@ export default {
     monitorRoleUser: 'Benutzer',
     monitorRoleAssistant: 'Assistent',
     copySessionId: 'Sitzungs-ID kopieren',
+    export: 'Exportieren',
+    exportFull: 'Vollständiger Export (JSON)',
+    exportCompressed: 'Komprimierter Export (TXT)',
+    exportCompressing: 'Komprimiere Kontext, bitte warten...',
+    exportSuccess: 'Sitzung exportiert',
+    exportFailed: 'Export fehlgeschlagen',
     renamed: 'Umbenannt',
     renameFailed: 'Umbenennung fehlgeschlagen',
     renameSession: 'Sitzung umbenennen',