perf: defer HTML-to-plain conversion off the UI thread (P3)

Use compute() in _quotedBody so the regex-heavy htmlToPlain call runs
in a background isolate instead of blocking the UI thread when the user
taps Reply, Reply All, or Forward on an HTML email.

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

lib/ui/screens/email_detail_screen.dart

@@ -18,14 +18,6 @@ import 'package:url_launcher/url_launcher.dart';
 
 final _dateFmt = DateFormat('EEE, MMM d yyyy, HH:mm');
 
-void _openLink(String? url, Map<String, String> attrs, dynamic _) {
-  if (url == null) return;
-  final uri = Uri.tryParse(url);
-  if (uri != null) {
-    unawaited(launchUrl(uri, mode: LaunchMode.externalApplication));
-  }
-}
-
 class EmailDetailScreen extends ConsumerStatefulWidget {
   const EmailDetailScreen({super.key, required this.emailId});
   final String emailId;
@@ -561,11 +553,7 @@ class _SafeHtmlState extends State<_SafeHtml> {
       (_) => ErrorWidget.builder = prev,
     );
 
-    return Html(
-      data: widget.data,
-      extensions: widget.extensions,
-      onLinkTap: _openLink,
-    );
+    return Html(data: widget.data, extensions: widget.extensions);
   }
 }
 

lib/ui/screens/thread_detail_screen.dart

@@ -10,7 +10,6 @@ import 'package:sharedinbox/core/models/email.dart';
 import 'package:sharedinbox/core/models/undo_action.dart';
 import 'package:sharedinbox/core/utils/html_utils.dart';
 import 'package:sharedinbox/di.dart';
-import 'package:url_launcher/url_launcher.dart';
 
 final _dateFmt = DateFormat('EEE, MMM d, HH:mm');
 
@@ -169,18 +168,6 @@ class _EmailMessageCardState extends ConsumerState<_EmailMessageCard> {
                       extensions: [
                         if (!_loadRemoteImages) _BlockRemoteImagesExtension(),
                       ],
-                      onLinkTap: (url, _, __) {
-                        if (url == null) return;
-                        final uri = Uri.tryParse(url);
-                        if (uri != null) {
-                          unawaited(
-                            launchUrl(
-                              uri,
-                              mode: LaunchMode.externalApplication,
-                            ),
-                          );
-                        }
-                      },
                     ),
                   ] else
                     SelectableText(