feat(i18n): remove orphaned conversation message keys

Drop the 22 message keys that only the deleted Briefwechsel view used
(conv_* except the still-used conv_sort_newest/oldest, plus
nav_conversations, doc_conversation_title and person_correspondents_hint,
all now superseded by the retargeted card's new search keys).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/messages/en.json

@@ -26,7 +26,6 @@
 	"error_internal_error": "An unexpected error occurred.",
 	"nav_documents": "Documents",
 	"nav_persons": "Persons",
-	"nav_conversations": "Letters",
 	"nav_admin": "Admin",
 	"nav_logout": "Sign out",
 	"layout_menu_open": "Open menu",
@@ -189,32 +188,12 @@
 	"person_role_sender": "Sent",
 	"person_role_receiver": "Received",
 	"person_co_correspondents_heading": "Frequent correspondents",
-	"person_correspondents_hint": "click to view conversation",
 	"person_correspondents_search_title": "Search letters from {A} to {B}",
 	"person_correspondents_search_hint": "click to search letters",
 	"person_correspondents_badge_title": "Shared letters in both directions",
 	"person_show_more": "+ {count} more",
-	"conv_label_person_a": "Person A (Sender)",
-	"conv_label_person_b": "Correspondent",
-	"conv_label_from": "Period from",
-	"conv_label_to": "Period to",
-	"conv_sort_label": "Sort:",
 	"conv_sort_newest": "Newest first",
 	"conv_sort_oldest": "Oldest first",
-	"conv_empty_heading": "Whose letters would you like to read?",
-	"conv_hero_crosslink": "Looking for a specific document? → Go to document search",
-	"conv_no_results_heading": "No documents found.",
-	"conv_no_results_text": "Try adjusting the time period.",
-	"conv_swap_btn": "Swap persons",
-	"conv_new_doc_link": "New document in this exchange",
-	"conv_strip_sort_newest": "Newest",
-	"conv_strip_sort_oldest": "Oldest",
-	"conv_suggestions_heading": "Top correspondents",
-	"conv_suggestions_all_label": "All correspondents of {name}",
-	"conv_letters_count": "{count} letters",
-	"conv_hero_divider": "or",
-	"conv_empty_recent_label": "Recently opened",
-	"conv_no_party": "—",
 	"dist_bar_segment": "{count} from {name}",
 	"dist_bar_aria": "Letter distribution in this period: {outCount} from {senderName}, {inCount} from {receiverName}",
 	"row_direction_sent": "Sent",
@@ -295,7 +274,6 @@
 	"topbar_overflow_heading": "More receivers",
 	"topbar_overflow_show": "Show {count} more receivers",
 	"doc_tag_filter_title": "Filter by {name}",
-	"doc_conversation_title": "Show conversation",
 	"doc_preview_iframe_title": "Document Preview",
 	"doc_image_alt": "Original scan",
 	"doc_no_date": "No date",