feat: add 'Create new folder' option to Move To Folder dialog

Adds a 'Create new folder…' tile at the bottom of the move-to
bottom sheet in the single mail view. Tapping it shows a text
input dialog; on confirm, the folder is created on the server
and the email is immediately moved there.

- Extend MailboxRepository with createMailbox(accountId, name)
- Implement it in MailboxRepositoryImpl for IMAP and JMAP,
  reusing existing private helpers with role made nullable
- Wire up the new flow in _moveTo() on EmailDetailScreen

Closes #422

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

lib/core/repositories/mailbox_repository.dart

@@ -20,4 +20,8 @@ abstract class MailboxRepository {
     String name,
     String role,
   );
+
+  /// Creates a new mailbox named [name] for [accountId] without a special role.
+  /// Returns the newly created [Mailbox].
+  Future<Mailbox> createMailbox(String accountId, String name);
 }

lib/data/repositories/mailbox_repository_impl.dart

@@ -343,11 +343,23 @@ class MailboxRepositoryImpl implements MailboxRepository {
     }
   }
 
+  @override
+  Future<model.Mailbox> createMailbox(String accountId, String name) async {
+    final account = (await _accounts.getAccount(accountId))!;
+    final password = await _accounts.getPassword(accountId);
+    switch (account.type) {
+      case account_model.AccountType.imap:
+        return _createMailboxWithRoleImap(account, password, name, null);
+      case account_model.AccountType.jmap:
+        return _createMailboxWithRoleJmap(account, password, name, null);
+    }
+  }
+
   Future<model.Mailbox> _createMailboxWithRoleImap(
     account_model.Account account,
     String password,
     String name,
-    String role,
+    String? role,
   ) async {
     final client = await _imapConnect(
       account,
@@ -380,7 +392,7 @@ class MailboxRepositoryImpl implements MailboxRepository {
     account_model.Account account,
     String password,
     String name,
-    String role,
+    String? role,
   ) async {
     final jmapUrl = account.jmapUrl;
     if (jmapUrl == null || jmapUrl.isEmpty) {
@@ -398,7 +410,10 @@ class MailboxRepositoryImpl implements MailboxRepository {
         {
           'accountId': jmap.accountId,
           'create': {
-            'new-mailbox': {'name': name, 'role': role},
+            'new-mailbox': {
+                'name': name,
+                if (role != null) 'role': role,
+              },
           },
         },
         '0',

lib/ui/screens/email_detail_screen.dart

@@ -551,6 +551,42 @@ class _EmailDetailScreenState extends ConsumerState<EmailDetailScreen> {
     );
   }
 
+  Future<String?> _promptNewFolderName(BuildContext context) async {
+    final controller = TextEditingController();
+    try {
+      return await showDialog<String>(
+        context: context,
+        builder: (ctx) => AlertDialog(
+          title: const Text('Create new folder'),
+          content: TextField(
+            controller: controller,
+            autofocus: true,
+            decoration: const InputDecoration(hintText: 'Folder name'),
+            textCapitalization: TextCapitalization.words,
+            onSubmitted: (value) {
+              if (value.trim().isNotEmpty) Navigator.pop(ctx, value.trim());
+            },
+          ),
+          actions: [
+            TextButton(
+              onPressed: () => Navigator.pop(ctx),
+              child: const Text('Cancel'),
+            ),
+            FilledButton(
+              onPressed: () {
+                final name = controller.text.trim();
+                if (name.isNotEmpty) Navigator.pop(ctx, name);
+              },
+              child: const Text('Create'),
+            ),
+          ],
+        ),
+      );
+    } finally {
+      controller.dispose();
+    }
+  }
+
   Future<void> _moveTo(BuildContext context, Email header) async {
     final nextEmailId = await _getNextEmailIdIfNeeded(header);
 
@@ -564,6 +600,8 @@ class _EmailDetailScreenState extends ConsumerState<EmailDetailScreen> {
 
     if (!context.mounted) return;
 
+    const createNewSentinel = '__create_new__';
+
     final chosen = await showModalBottomSheet<String>(
       context: context,
       builder: (ctx) => ListView(
@@ -581,13 +619,28 @@ class _EmailDetailScreenState extends ConsumerState<EmailDetailScreen> {
               title: Text(m.name),
               onTap: () => Navigator.pop(ctx, m.path),
             ),
+          ListTile(
+            leading: const Icon(Icons.create_new_folder_outlined),
+            title: const Text('Create new folder…'),
+            onTap: () => Navigator.pop(ctx, createNewSentinel),
+          ),
         ],
       ),
     );
 
     if (chosen == null || !context.mounted) return;
 
-    await ref.read(emailRepositoryProvider).moveEmail(widget.emailId, chosen);
+    String destination = chosen;
+    if (chosen == createNewSentinel) {
+      final name = await _promptNewFolderName(context);
+      if (name == null || !context.mounted) return;
+      final mailbox = await mailboxRepo.createMailbox(header.accountId, name);
+      destination = mailbox.path;
+    }
+
+    await ref
+        .read(emailRepositoryProvider)
+        .moveEmail(widget.emailId, destination);
 
     unawaited(
       ref.read(undoServiceProvider.notifier).pushAction(
@@ -597,7 +650,7 @@ class _EmailDetailScreenState extends ConsumerState<EmailDetailScreen> {
               type: UndoType.move,
               emailIds: [widget.emailId],
               sourceMailboxPath: header.mailboxPath,
-              destinationMailboxPath: chosen,
+              destinationMailboxPath: destination,
             ),
           ),
     );