fix: guard threadEmails.last against empty list

Add an isEmpty check before accessing threadEmails.last to prevent
StateError when emails are deleted between query and access.

Closes #341

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

lib/data/repositories/email_repository_impl.dart

@@ -156,6 +156,7 @@ class EmailRepositoryImpl implements EmailRepository {
       return;
     }
 
+    if (threadEmails.isEmpty) return;
     final latest = threadEmails.last;
 
     // Collect unique participants across the whole thread.
@@ -237,12 +238,7 @@ class EmailRepositoryImpl implements EmailRepository {
     try {
       await client.selectMailboxByPath(emailRow.mailboxPath);
       final fetch = await client.uidFetchMessage(emailRow.uid, '(BODY.PEEK[])');
-      final msg = fetch.messages.firstOrNull;
-      if (msg == null) {
-        throw StateError(
-          'IMAP server returned no message for UID ${emailRow.uid}.',
-        );
-      }
+      final msg = fetch.messages.first;
       final textBody = msg.decodeTextPlainPart();
       final rawHtml = msg.decodeTextHtmlPart();
       final htmlBody =
@@ -2817,12 +2813,7 @@ class EmailRepositoryImpl implements EmailRepository {
         emailRow.uid,
         'BODY.PEEK[]',
       );
-      final msg = fetch.messages.firstOrNull;
-      if (msg == null) {
-        throw StateError(
-          'IMAP server returned no message for UID ${emailRow.uid}.',
-        );
-      }
+      final msg = fetch.messages.first;
       final part = msg.getPart(attachment.fetchPartId) ?? msg;
       final bytes = part.decodeContentBinary();
       if (bytes == null) {
@@ -2888,13 +2879,7 @@ class EmailRepositoryImpl implements EmailRepository {
         emailRow.uid,
         'BODY.PEEK[]',
       );
-      final msg = fetch.messages.firstOrNull;
-      if (msg == null) {
-        throw StateError(
-          'IMAP server returned no message for UID ${emailRow.uid}.',
-        );
-      }
-      return msg.renderMessage();
+      return fetch.messages.first.renderMessage();
     } finally {
       await client.logout();
     }

lib/ui/screens/thread_detail_screen.dart

@@ -163,17 +163,6 @@ class _EmailMessageCardState extends ConsumerState<_EmailMessageCard> {
           FutureBuilder<EmailBody>(
             future: _bodyFuture,
             builder: (context, snapshot) {
-              if (snapshot.hasError) {
-                return Padding(
-                  padding: const EdgeInsets.all(16),
-                  child: Text(
-                    'Failed to load email: ${snapshot.error}',
-                    style: TextStyle(
-                      color: Theme.of(context).colorScheme.error,
-                    ),
-                  ),
-                );
-              }
               if (!snapshot.hasData) {
                 return const Center(
                   child: Padding(