fix(security): treat a new identity key as a message request, never as a known contact

A stranger could pick the display name of an existing contact ("Mom")
and appear as that contact. Messages from a key that is not in the
contact list now open a message request labelled "(unknown)": no
replies, read receipts, typing indicators, calls or name in
notifications until the user accepts it and names the contact
themselves. Names that look like an existing contact (case, diacritics,
Cyrillic/Greek homoglyphs, invisible characters) also show a warning in
the real contact's chat.

When the relay account of a known contact shows up with a different
key, the chat shows "safety number changed", verification is cleared
and the session that now belongs to the other key is dropped before
sending, so a reply never goes to the new key.

app/lib/features/chat_list/archived_chats_screen.dart

@@ -1,6 +1,7 @@
 import 'package:flutter/material.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 import 'package:go_router/go_router.dart';
+import 'package:sepot/core/l10n/contact_name.dart';
 import 'package:sepot/core/l10n/error_messages.dart';
 import 'package:sepot/core/l10n/l10n.dart';
 import 'package:sepot/core/theme/app_theme.dart';
@@ -45,13 +46,15 @@ class ArchivedChatsScreen extends ConsumerWidget {
                   ?.where((c) => c.publicIdentity.id == conv.contactId)
                   .firstOrNull;
               final name = group?.name ??
-                  contact?.displayName ??
-                  context.l10n.commonUnknownContact;
+                  (contact == null
+                      ? context.l10n.commonUnknownContact
+                      : contactName(context.l10n, contact));
               return ChatListTile(
                 conversation: conv,
                 contactName: name,
                 myId: myId,
                 isGroup: group != null,
+                isRequest: group == null && contact?.accepted == false,
                 onTap: () => context.push('/conversation/${conv.contactId}'),
                 onLongPress: () => _actions(context, ref, conv),
               );

app/lib/features/chat_list/chat_list_screen.dart

@@ -2,6 +2,7 @@ import 'package:flutter/material.dart';
 import 'package:flutter/services.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 import 'package:go_router/go_router.dart';
+import 'package:sepot/core/l10n/contact_name.dart';
 import 'package:sepot/core/l10n/error_messages.dart';
 import 'package:sepot/core/l10n/l10n.dart';
 import 'package:sepot/core/theme/app_theme.dart';
@@ -206,7 +207,7 @@ class _ChatsTabState extends ConsumerState<_ChatsTab> {
     String nameOf(Conversation c) => ConversationsNotifier.isNotes(c.contactId)
         ? l10n.notesTitle
         : groupOf(c.contactId)?.name ??
-        contactOf(c.contactId)?.displayName ??
+        _nameOrNull(l10n, contactOf(c.contactId)) ??
         l10n.commonUnknownContact;
 
     return SafeArea(
@@ -265,7 +266,8 @@ class _ChatsTabState extends ConsumerState<_ChatsTab> {
                     ? l10n.chatTyping
                     : writers.length == 1
                         ? l10n.chatTypingNamed(
-                            (contactOf(writers.first)?.displayName ?? '?')
+                            (_nameOrNull(l10n, contactOf(writers.first)) ??
+                                    '?')
                                 .split(' ')
                                 .first)
                         : l10n.chatTypingSeveral;
@@ -275,8 +277,10 @@ class _ChatsTabState extends ConsumerState<_ChatsTab> {
               myId: myId,
               isGroup: group != null,
               verified: contactOf(c.contactId)?.verified ?? false,
+              isRequest: group == null &&
+                  contactOf(c.contactId)?.accepted == false,
               lastAuthor: group != null && last != null
-                  ? contactOf(last.senderId)?.displayName
+                  ? _nameOrNull(l10n, contactOf(last.senderId))
                   : null,
               highlight: hits[c],
               typingLabel: typingLabel,
@@ -685,3 +689,6 @@ class _EmptyState extends StatelessWidget {
     );
   }
 }
+
+String? _nameOrNull(AppLocalizations l10n, Contact? c) =>
+    c == null ? null : contactName(l10n, c);

app/lib/features/chat_list/chat_list_tile.dart

@@ -18,6 +18,10 @@ class ChatListTile extends StatelessWidget {
   final bool isGroup;
   final bool verified;
 
+  /// Žádost o zprávu (neznámý odesílatel): náhled místo textu hlásí, o co
+  /// jde — první věta útočníka nemá dostat prostor v seznamu chatů.
+  final bool isRequest;
+
   /// Jméno autora poslední zprávy ve skupině (null = 1:1 nebo neznámý).
   final String? lastAuthor;
 
@@ -40,6 +44,7 @@ class ChatListTile extends StatelessWidget {
     required this.myId,
     this.isGroup = false,
     this.verified = false,
+    this.isRequest = false,
     this.lastAuthor,
     this.highlight,
     this.typingLabel,
@@ -54,7 +59,9 @@ class ChatListTile extends StatelessWidget {
     final unread = conversation.unreadCount(myId);
     final muted = conversation.isMuted;
     final mine = lastMsg != null && lastMsg.isMine(myId);
-    final preview = previewOf(context.l10n, lastMsg);
+    final preview = isRequest
+        ? (icon: Icons.person_search_rounded, text: context.l10n.requestTitle)
+        : previewOf(context.l10n, lastMsg);
 
     return InkWell(
       onTap: onTap,
@@ -245,6 +252,9 @@ class ChatListTile extends StatelessWidget {
       final label = callLogLabel(l10n, call);
       return (icon: label.icon, text: label.text);
     }
+    if (msg.notice != null) {
+      return (icon: Icons.gpp_maybe_rounded, text: l10n.noticePreview);
+    }
     return switch (msg.type) {
       MessageType.text => (icon: null, text: msg.content ?? ''),
       MessageType.image => (