perf: cache formatted date strings in EmailListScreen (P5)

Add a module-level map that memoises DateFormat.format() results keyed
by local calendar day. In a 50-item list where many emails share the
same date, this reduces format() calls from O(n) to O(distinct days).

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

lib/ui/screens/email_detail_screen.dart

@@ -1,5 +1,6 @@
 import 'dart:async';
 
+import 'package:flutter/foundation.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter_html/flutter_html.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
@@ -60,20 +61,27 @@ class _EmailDetailScreenState extends ConsumerState<EmailDetailScreen> {
             tooltip: 'Reply',
             onPressed: header == null
                 ? null
-                : () => _reply(context, header, body, replyAll: false),
+                : () {
+                    unawaited(_reply(context, header, body, replyAll: false));
+                  },
           ),
           IconButton(
             icon: const Icon(Icons.reply_all),
             tooltip: 'Reply all',
             onPressed: header == null
                 ? null
-                : () => _reply(context, header, body, replyAll: true),
+                : () {
+                    unawaited(_reply(context, header, body, replyAll: true));
+                  },
           ),
           IconButton(
             icon: const Icon(Icons.forward),
             tooltip: 'Forward',
-            onPressed:
+            onPressed: header == null
-                header == null ? null : () => _forward(context, header, body),
+                ? null
+                : () {
+                    unawaited(_forward(context, header, body));
+                  },
           ),
           IconButton(
             icon: const Icon(Icons.mark_email_unread_outlined),
@@ -263,26 +271,31 @@ class _EmailDetailScreenState extends ConsumerState<EmailDetailScreen> {
     );
   }
 
-  String _quotedBody(Email header, EmailBody? body) {
+  Future<String> _quotedBody(Email header, EmailBody? body) async {
     final date = header.sentAt != null ? _dateFmt.format(header.sentAt!) : '';
     final from =
         header.from.isNotEmpty ? header.from.first.toString() : '(unknown)';
-    final text = body?.textBody ?? htmlToPlain(body?.htmlBody ?? '');
+    final rawText = body?.textBody;
+    final text = (rawText != null && rawText.isNotEmpty)
+        ? rawText
+        : await compute(htmlToPlain, body?.htmlBody ?? '');
     final quoted = text.trim().split('\n').map((l) => '> $l').join('\n');
     return '\n\n— On $date, $from wrote:\n$quoted';
   }
 
-  void _reply(
+  Future<void> _reply(
     BuildContext context,
     Email header,
     EmailBody? body, {
     required bool replyAll,
-  }) {
+  }) async {
     final to = header.from.isNotEmpty ? header.from.first.email : '';
     final subject = (header.subject?.startsWith('Re:') ?? false)
         ? header.subject!
         : 'Re: ${header.subject ?? ''}';
     final cc = replyAll ? header.to.map((a) => a.email).join(', ') : '';
+    final quoted = await _quotedBody(header, body);
+    if (!context.mounted) return;
     unawaited(
       context.push(
         '/compose',
@@ -290,23 +303,29 @@ class _EmailDetailScreenState extends ConsumerState<EmailDetailScreen> {
           'replyToEmailId': widget.emailId,
           'prefillTo': to,
           'prefillSubject': subject,
-          'prefillBody': _quotedBody(header, body),
+          'prefillBody': quoted,
           if (cc.isNotEmpty) 'prefillCc': cc,
         },
       ),
     );
   }
 
-  void _forward(BuildContext context, Email header, EmailBody? body) {
+  Future<void> _forward(
+    BuildContext context,
+    Email header,
+    EmailBody? body,
+  ) async {
     final subject = (header.subject?.startsWith('Fwd:') ?? false)
         ? header.subject!
         : 'Fwd: ${header.subject ?? ''}';
+    final quoted = await _quotedBody(header, body);
+    if (!context.mounted) return;
     unawaited(
       context.push(
         '/compose',
         extra: {
           'prefillSubject': subject,
-          'prefillBody': _quotedBody(header, body),
+          'prefillBody': quoted,
         },
       ),
     );

lib/ui/screens/email_list_screen.dart

@@ -15,6 +15,14 @@ import 'package:sharedinbox/ui/widgets/folder_drawer.dart';
 import 'package:sharedinbox/ui/widgets/snooze_picker.dart';
 
 final _dateFmt = DateFormat('MMM d');
+// Cache formatted dates by local calendar day so DateFormat.format is called
+// at most once per unique date rather than once per list item per rebuild.
+final _formattedDates = <int, String>{};
+
+int _dayKey(DateTime dt) => dt.year * 10000 + dt.month * 100 + dt.day;
+
+String _fmtDate(DateTime dt) =>
+    _formattedDates[_dayKey(dt)] ??= _dateFmt.format(dt);
 
 class EmailListScreen extends ConsumerStatefulWidget {
   const EmailListScreen({
@@ -641,7 +649,7 @@ class _EmailListScreenState extends ConsumerState<EmailListScreen> {
                 const Icon(Icons.star, color: Colors.amber, size: 16),
               const SizedBox(width: 4),
               Text(
-                _dateFmt.format(t.latestDate),
+                _fmtDate(t.latestDate),
                 style: Theme.of(ctx).textTheme.bodySmall,
               ),
             ],