fix: guard against empty IMAP fetch message list (#339)

Replace unsafe .first on fetch.messages with .firstOrNull + early throw in
getEmailBody, downloadAttachment, and fetchRawRfc822, so a UID-expunged or
network-hiccup response gives a descriptive StateError instead of the
opaque "No element" crash. Also add snapshot.hasError handling in the
FutureBuilder so users see an error message rather than an infinite spinner.

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

lib/data/repositories/email_repository_impl.dart

@@ -237,7 +237,12 @@ class EmailRepositoryImpl implements EmailRepository {
     try {
       await client.selectMailboxByPath(emailRow.mailboxPath);
       final fetch = await client.uidFetchMessage(emailRow.uid, '(BODY.PEEK[])');
-      final msg = fetch.messages.first;
+      final msg = fetch.messages.firstOrNull;
+      if (msg == null) {
+        throw StateError(
+          'IMAP server returned no message for UID ${emailRow.uid}.',
+        );
+      }
       final textBody = msg.decodeTextPlainPart();
       final rawHtml = msg.decodeTextHtmlPart();
       final htmlBody =
@@ -2812,7 +2817,12 @@ class EmailRepositoryImpl implements EmailRepository {
         emailRow.uid,
         'BODY.PEEK[]',
       );
-      final msg = fetch.messages.first;
+      final msg = fetch.messages.firstOrNull;
+      if (msg == null) {
+        throw StateError(
+          'IMAP server returned no message for UID ${emailRow.uid}.',
+        );
+      }
       final part = msg.getPart(attachment.fetchPartId) ?? msg;
       final bytes = part.decodeContentBinary();
       if (bytes == null) {
@@ -2878,7 +2888,13 @@ class EmailRepositoryImpl implements EmailRepository {
         emailRow.uid,
         'BODY.PEEK[]',
       );
-      return fetch.messages.first.renderMessage();
+      final msg = fetch.messages.firstOrNull;
+      if (msg == null) {
+        throw StateError(
+          'IMAP server returned no message for UID ${emailRow.uid}.',
+        );
+      }
+      return msg.renderMessage();
     } finally {
       await client.logout();
     }

lib/ui/screens/thread_detail_screen.dart

@@ -163,6 +163,17 @@ 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(