fix(mobile): stop cutting previews and transcripts through an emoji

mobile/lib/features/channels/agent_activity/transcript_item_widget.dart

@@ -5,6 +5,7 @@ import 'package:flutter_hooks/flutter_hooks.dart';
 import 'package:gpt_markdown/gpt_markdown.dart';
 import 'package:lucide_icons_flutter/lucide_icons.dart';
 
+import '../../../shared/text/truncate.dart';
 import '../../../shared/theme/theme.dart';
 import 'observer_models.dart';
 
@@ -245,9 +246,7 @@ class _MetadataItemWidget extends HookWidget {
                   if (section.body.isNotEmpty) ...[
                     const SizedBox(height: Grid.quarter),
                     Text(
-                      section.body.length > 500
-                          ? '${section.body.substring(0, 500)}\u2026'
-                          : section.body,
+                      truncateWithEllipsis(section.body, 500, '\u2026'),
                       style: context.textTheme.bodySmall?.copyWith(
                         color: context.colors.onSurfaceVariant,
                       ),
@@ -401,9 +400,11 @@ class _ToolItemWidget extends HookWidget {
               if (resultExpanded.value) ...[
                 const SizedBox(height: Grid.quarter),
                 _CodeBlock(
-                  text: item.result.length > 2000
-                      ? '${item.result.substring(0, 2000)}\n\n\u2026 (truncated)'
-                      : item.result,
+                  text: truncateWithEllipsis(
+                    item.result,
+                    2000,
+                    '\n\n\u2026 (truncated)',
+                  ),
                   isError: item.isError,
                 ),
               ],

mobile/lib/features/forum/forum_post_card.dart

@@ -5,6 +5,7 @@ import 'package:hooks_riverpod/hooks_riverpod.dart';
 import 'package:lucide_icons_flutter/lucide_icons.dart';
 
 import '../../shared/mentions/agent_identity_provider.dart';
+import '../../shared/text/truncate.dart';
 import '../../shared/theme/theme.dart';
 import '../../shared/widgets/avatar_image.dart';
 import '../../shared/widgets/modal_presentation.dart';
@@ -85,9 +86,7 @@ class ForumPostCard extends HookConsumerWidget {
       directoryDisplayNames: ref.watch(agentDirectoryDisplayNamesProvider),
       agentMentionPubkeys: agentMentionPubkeys,
     );
-    final preview = post.content.length > 200
-        ? '${post.content.substring(0, 200)}...'
-        : post.content;
+    final preview = truncateWithEllipsis(post.content, 200, '...');
     final summary = post.threadSummary;
 
     return GestureDetector(

mobile/lib/shared/text/truncate.dart

@@ -0,0 +1,37 @@
+/// Truncation for text that is shown to a person.
+library;
+
+import 'package:characters/characters.dart';
+
+/// Returns [text] shortened to at most [maxCharacters] user-perceived
+/// characters, or [text] unchanged when it is already that short.
+///
+/// `substring` and `length` count UTF-16 code units. Anything outside the
+/// Basic Multilingual Plane — every emoji, and CJK Extension B — is stored as
+/// a surrogate pair, so a cut that lands inside one leaves a lone surrogate:
+/// not a character, and drawn as `\u{FFFD}` at the end of the preview.
+///
+/// Counting characters also means the decision to truncate and the cut itself
+/// agree with each other, and with what the reader sees.
+///
+/// The reminder preview in `features/channels/message_actions.dart` already
+/// does this with `.characters.take(...)`; this is the same rule, shared.
+String truncateToCharacters(String text, int maxCharacters) {
+  if (maxCharacters <= 0) return '';
+  final characters = text.characters;
+  if (characters.length <= maxCharacters) return text;
+  return characters.take(maxCharacters).toString();
+}
+
+/// Returns [text] with [ellipsis] appended when it had to be shortened to
+/// [maxCharacters], or [text] unchanged when it fits.
+///
+/// Callers used to spell this out as
+/// `text.length > n ? '${text.substring(0, n)}…' : text`, which counts code
+/// units twice over — once to decide, once to cut.
+String truncateWithEllipsis(String text, int maxCharacters, String ellipsis) {
+  final truncated = truncateToCharacters(text, maxCharacters);
+  // `truncateToCharacters` returns `text` itself when it fits, so comparing
+  // lengths is exact and avoids walking the string again.
+  return truncated.length == text.length ? text : '$truncated$ellipsis';
+}

mobile/test/shared/text/truncate_test.dart

@@ -0,0 +1,39 @@
+import 'package:buzz/shared/text/truncate.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  test('leaves text that is already short enough', () {
+    expect(truncateToCharacters('hello', 100), 'hello');
+    expect(truncateToCharacters('hello', 5), 'hello');
+  });
+
+  test('never ends on half a surrogate pair', () {
+    final body = '${'x' * 199}\u{1F389} more text';
+    final cut = truncateToCharacters(body, 200);
+    expect(cut, '${'x' * 199}\u{1F389}');
+    // A lone surrogate would leave the last code unit in D800..DBFF.
+    expect(cut.codeUnitAt(cut.length - 1) & 0xFC00 == 0xD800, isFalse);
+  });
+
+  test('counts characters, not code units', () {
+    const party = '\u{1F389}';
+    expect(truncateToCharacters(party * 3, 2), party * 2);
+    expect(truncateToCharacters(party * 3, 2).runes.length, 2);
+  });
+
+  test('handles the degenerate limits', () {
+    expect(truncateToCharacters('hello', 0), '');
+    expect(truncateToCharacters('', 10), '');
+  });
+
+  test('appends the ellipsis only when it actually cut', () {
+    expect(truncateWithEllipsis('hello', 100, '...'), 'hello');
+    expect(truncateWithEllipsis('hello', 5, '...'), 'hello');
+    expect(truncateWithEllipsis('hello there', 5, '...'), 'hello...');
+  });
+
+  test('does not cut the ellipsis into an emoji', () {
+    final body = '${'x' * 199}\u{1F389} more text';
+    expect(truncateWithEllipsis(body, 200, '...'), '${'x' * 199}\u{1F389}...');
+  });
+}