feat(conversations): add swap button (#32)

Adds a button between the two person typeaheads that swaps sender and
receiver, then reloads the conversation view.

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

frontend/messages/de.json

@@ -140,6 +140,7 @@
 	"conv_empty_text": "Die Korrespondenz wird hier angezeigt.",
 	"conv_no_results_heading": "Keine Dokumente gefunden.",
 	"conv_no_results_text": "Versuchen Sie, den Zeitraum anzupassen.",
+	"conv_swap_btn": "Personen tauschen",
 
 	"admin_heading": "Admin Dashboard",
 	"admin_tab_users": "Benutzer",