fix(mobile): draw a whole character in an avatar fallback

mobile/lib/features/channels/channel_management_provider.dart

@@ -9,6 +9,7 @@ import '../../shared/custom_emoji/custom_emoji.dart';
 import '../../shared/custom_emoji/custom_emoji_provider.dart';
 import '../../shared/mentions/agent_identity_provider.dart';
 import '../../shared/relay/relay.dart';
+import '../../shared/text/initial.dart';
 import '../profile/profile_provider.dart';
 import 'channel.dart';
 import 'channels_provider.dart';
@@ -178,7 +179,7 @@ class DirectoryUser {
   }
 
   /// First visible character used when no avatar image is available.
-  String get initial => label.isNotEmpty ? label[0].toUpperCase() : '?';
+  String get initial => avatarInitial(label);
 }
 
 /// Whether the mobile DM directory should show local preview identities.

mobile/lib/features/channels/channels_page.dart

@@ -14,6 +14,7 @@ import 'package:lucide_icons_flutter/lucide_icons.dart';
 import '../../shared/auth/auth.dart';
 import '../../shared/community/community_icon_provider.dart';
 import '../../shared/relay/relay.dart';
+import '../../shared/text/initial.dart';
 import '../../shared/theme/theme.dart';
 import '../../shared/widgets/avatar_image.dart';
 import '../../shared/widgets/anchored_popover_menu.dart';

mobile/lib/features/channels/channels_page/community.dart

@@ -489,9 +489,7 @@ class _CommunityAvatar extends ConsumerWidget {
   @override
   Widget build(BuildContext context, WidgetRef ref) {
     final trimmedName = name?.trim();
-    final initial = trimmedName != null && trimmedName.isNotEmpty
-        ? trimmedName.substring(0, 1).toUpperCase()
-        : '?';
+    final initial = avatarInitial(trimmedName ?? '');
     final relay = relayUrl;
     final iconUrl = relay == null
         ? null

mobile/lib/features/channels/compose_bar.dart

@@ -19,6 +19,7 @@ import 'package:nostr/nostr.dart' as nostr;
 
 import '../../shared/mentions/agent_identity_provider.dart';
 import '../../shared/relay/relay.dart';
+import '../../shared/text/initial.dart';
 import '../../shared/theme/theme.dart';
 import '../../shared/widgets/avatar_image.dart';
 import '../../shared/widgets/anchored_popover_menu.dart';

mobile/lib/features/channels/compose_bar/suggestions.dart

@@ -47,7 +47,7 @@ class _MentionSuggestions extends StatelessWidget {
                 radius: 18,
                 backgroundColor: context.colors.primaryContainer,
                 fallback: Text(
-                  name[0].toUpperCase(),
+                  avatarInitial(name),
                   style: context.textTheme.labelMedium?.copyWith(
                     color: context.colors.onPrimaryContainer,
                     fontWeight: FontWeight.w600,

mobile/lib/features/channels/members_sheet.dart

@@ -3,6 +3,7 @@ import 'package:flutter_hooks/flutter_hooks.dart';
 import 'package:hooks_riverpod/hooks_riverpod.dart';
 import 'package:lucide_icons_flutter/lucide_icons.dart';
 
+import '../../shared/text/initial.dart';
 import '../../shared/theme/theme.dart';
 import '../../shared/widgets/avatar_image.dart';
 import '../../shared/widgets/buzz_loading_indicator.dart';
@@ -220,7 +221,7 @@ class _MemberTile extends ConsumerWidget {
         : (profile?.displayName?.trim().isNotEmpty == true
               ? profile!.displayName!.trim()
               : member.labelFor(currentPubkey));
-    final initial = label.substring(0, 1).toUpperCase();
+    final initial = avatarInitial(label);
     final showManagementActions = canManage && !isSelf && !member.isOwner;
     final showMenu = showManagementActions || onViewActivity != null;
 

mobile/lib/features/invites/invite_create_provider.dart

@@ -9,6 +9,7 @@ import 'package:share_plus/share_plus.dart';
 
 import '../../shared/community/community_membership_provider.dart';
 import '../../shared/relay/relay.dart';
+import '../../shared/text/initial.dart';
 
 /// The default lifetime of a newly minted community invite link.
 const defaultCommunityInviteTtlSeconds = 3 * 24 * 60 * 60;
@@ -123,7 +124,7 @@ class CommunityInviteDirectoryUser {
   }
 
   /// The uppercase first character of [label], or `?` when unavailable.
-  String get initial => label.isEmpty ? '?' : label[0].toUpperCase();
+  String get initial => avatarInitial(label);
 }
 
 /// Abbreviates a hexadecimal public key for compact display.

mobile/lib/features/profile/user_profile.dart

@@ -1,5 +1,7 @@
 import 'package:flutter/foundation.dart';
 
+import '../../shared/text/initial.dart';
+
 @immutable
 class UserProfile {
   final String pubkey;
@@ -36,8 +38,7 @@ class UserProfile {
 
   /// First letter for fallback avatar.
   String get initial =>
-      (displayName?.isNotEmpty == true ? displayName! : pubkey)[0]
-          .toUpperCase();
+      avatarInitial(displayName?.isNotEmpty == true ? displayName! : pubkey);
 }
 
 /// Optional profile handle shown beside a message author's display name.

mobile/lib/features/pulse/compose_note_page.dart

@@ -2,6 +2,7 @@ import 'package:flutter/material.dart';
 import 'package:flutter_hooks/flutter_hooks.dart';
 import 'package:hooks_riverpod/hooks_riverpod.dart';
 
+import '../../shared/text/initial.dart';
 import '../../shared/theme/theme.dart';
 import '../../shared/widgets/avatar_image.dart';
 import '../../shared/widgets/buzz_loading_indicator.dart';
@@ -191,7 +192,7 @@ class _ReplyContext extends ConsumerWidget {
                 radius: 18,
                 backgroundColor: context.colors.primaryContainer,
                 fallback: Text(
-                  (profile?.initial ?? displayName[0]).toUpperCase(),
+                  profile?.initial ?? avatarInitial(displayName),
                   style: context.textTheme.labelMedium?.copyWith(
                     color: context.colors.onPrimaryContainer,
                   ),

mobile/lib/features/pulse/note_card.dart

@@ -5,6 +5,7 @@ import 'package:lucide_icons_flutter/lucide_icons.dart';
 import 'package:nostr/nostr.dart' as nostr;
 
 import '../../shared/clipboard_utils.dart';
+import '../../shared/text/initial.dart';
 import '../../shared/theme/theme.dart';
 import '../../shared/widgets/avatar_image.dart';
 import '../channels/channel_detail_page.dart';
@@ -70,7 +71,7 @@ class NoteCard extends HookConsumerWidget {
               radius: 18,
               backgroundColor: context.colors.primaryContainer,
               fallback: Text(
-                (profile?.initial ?? displayName[0]).toUpperCase(),
+                profile?.initial ?? avatarInitial(displayName),
                 style: context.textTheme.labelMedium?.copyWith(
                   color: context.colors.onPrimaryContainer,
                 ),

mobile/lib/features/search/search_page.dart

@@ -6,6 +6,7 @@ import 'package:lucide_icons_flutter/lucide_icons.dart';
 
 import '../../shared/mentions/agent_identity_provider.dart';
 import '../../shared/mentions/mention_tags.dart';
+import '../../shared/text/initial.dart';
 import '../../shared/theme/theme.dart';
 import '../../shared/widgets/avatar_image.dart';
 import '../../shared/widgets/buzz_loading_indicator.dart';
@@ -729,7 +730,7 @@ class _PeopleSection extends ConsumerWidget {
               key: ValueKey('search-person-leading-${user.pubkey}'),
               imageUrl: user.avatarUrl,
               radius: 20,
-              fallback: Text(user.label.substring(0, 1).toUpperCase()),
+              fallback: Text(avatarInitial(user.label)),
             ),
             title: Text(
               user.label,

mobile/lib/shared/text/initial.dart

@@ -0,0 +1,26 @@
+/// The single character shown in an avatar when no image is available.
+library;
+
+import 'package:characters/characters.dart';
+
+/// Returns the first user-perceived character of [label], uppercased, or `?`
+/// when [label] has none.
+///
+/// `label[0]` and `label.substring(0, 1)` return one UTF-16 code unit.
+/// Anything outside the Basic Multilingual Plane — most emoji, and CJK
+/// Extension B, which appears in ordinary Chinese and Japanese given names —
+/// is stored as a surrogate pair, so those return half of one: not a
+/// character, and drawn as `\u{FFFD}` in the avatar. They also throw on an
+/// empty label; this returns `?`.
+///
+/// Grapheme clusters also keep a base letter together with its combining
+/// marks, so a Devanagari or Burmese name keeps its vowel sign instead of
+/// showing a bare consonant.
+///
+/// Mirrors desktop's `getInitials` (`desktop/src/shared/lib/initials.ts`) in
+/// treating text as characters rather than code units.
+String avatarInitial(String label) {
+  final characters = label.characters;
+  if (characters.isEmpty) return '?';
+  return characters.first.toUpperCase();
+}

mobile/test/shared/text/initial_test.dart

@@ -0,0 +1,30 @@
+import 'package:buzz/shared/text/initial.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  test('uppercases an ordinary first letter', () {
+    expect(avatarInitial('alice'), 'A');
+    expect(avatarInitial('Bravo Beta'), 'B');
+  });
+
+  test('falls back to ? for an empty label', () {
+    expect(avatarInitial(''), '?');
+  });
+
+  test('keeps a whole astral character instead of half a surrogate pair', () {
+    // U+20000, CJK Extension B — an ordinary character in some names.
+    const name = '\u{20000}\u{660E}';
+    final initial = avatarInitial(name);
+    expect(initial, '\u{20000}');
+    expect(initial.runes.length, 1);
+  });
+
+  test('keeps an emoji whole', () {
+    expect(avatarInitial('\u{1F389} party'), '\u{1F389}');
+  });
+
+  test('keeps a base letter together with its combining mark', () {
+    // Devanagari: the vowel sign belongs to the consonant before it.
+    expect(avatarInitial('\u{0928}\u{093F}\u{0932}'), '\u{0928}\u{093F}');
+  });
+}