feat(#145): switch dashboard to show last-activity documents

Replace recent-by-creation fetch with GET /api/documents/recent-activity
(sorted by updatedAt) in the dashboard. Update DashboardRecentDocuments
component to use doc.updatedAt, update i18n heading to "Zuletzt aktiv" /
"Recent Activity" / "Actividad reciente", and regenerate API types.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/messages/de.json

@@ -318,7 +318,7 @@
 	"dashboard_notification_replied": "hat geantwortet",
 	"dashboard_needs_metadata_heading": "Metadaten fehlen",
 	"dashboard_needs_metadata_show_all": "Alle anzeigen",
-	"dashboard_recent_heading": "Zuletzt hinzugefügt",
+	"dashboard_recent_heading": "Zuletzt aktiv",
 	"dashboard_resume_label": "Zuletzt geöffnet:",
 	"dashboard_resume_fallback": "Unbekanntes Dokument"
 }