feat(conversations): add new document link pre-filled with both persons (#33)

Adds a link next to the summary that navigates to the new-document form
with senderId and receiverId pre-filled from the current conversation.

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

frontend/messages/es.json

@@ -142,6 +142,7 @@
 	"conv_no_results_text": "Intente ajustar el período de tiempo.",
 	"conv_swap_btn": "Intercambiar personas",
 	"conv_summary": "{count} documentos · {yearFrom}–{yearTo}",
+	"conv_new_doc_link": "Nuevo documento en esta correspondencia",
 
 	"admin_heading": "Panel de administración",
 	"admin_tab_users": "Usuarios",