feat(i18n): rename Korrespondenz → Briefwechsel in all languages

Update nav label, page heading, empty-state headline, and document
link text. German uses "Briefwechsel", English "Letters", Spanish
"Cartas". Empty-state headline now uses the discovery framing from the
design discussion.

Refs: #179

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

frontend/messages/en.json

@@ -16,7 +16,7 @@
 	"error_internal_error": "An unexpected error occurred.",
 	"nav_documents": "Documents",
 	"nav_persons": "Persons",
-	"nav_conversations": "Correspondence",
+	"nav_conversations": "Letters",
 	"nav_admin": "Admin",
 	"nav_logout": "Sign out",
 	"btn_save": "Save",
@@ -130,7 +130,7 @@
 	"person_co_correspondents_heading": "Frequent correspondents",
 	"person_correspondents_hint": "click to view conversation",
 	"person_show_more": "+ {count} more",
-	"conv_heading": "Correspondence",
+	"conv_heading": "Letters",
 	"conv_subtitle": "Browse a person's letters — with or without a correspondent.",
 	"conv_label_person_a": "Person A (Sender)",
 	"conv_label_person_b": "Correspondent",
@@ -139,13 +139,13 @@
 	"conv_sort_label": "Sort:",
 	"conv_sort_newest": "Newest first",
 	"conv_sort_oldest": "Oldest first",
-	"conv_empty_heading": "Browse correspondence",
+	"conv_empty_heading": "Whose letters would you like to read?",
 	"conv_empty_text": "Choose a person from the archive to see their letters — with or without a correspondent.",
 	"conv_no_results_heading": "No documents found.",
 	"conv_no_results_text": "Try adjusting the time period.",
 	"conv_swap_btn": "Swap persons",
 	"conv_summary": "{count} documents · {yearFrom}–{yearTo}",
-	"conv_new_doc_link": "New document in this correspondence",
+	"conv_new_doc_link": "New document in this exchange",
 	"conv_label_correspondent_optional": "Correspondent",
 	"conv_hint_single_person": "All letters from {name} — choose a correspondent above to narrow down",
 	"conv_hint_single_person_filtered": "All letters from {name} · {from}–{to} · {sortLabel}",