feat: conversation grouping by default (Wino-style) + flatten single-mail threads

- MailListView: default group mode = GroupByThread (was None)
  - robustly applied at EmailTreeModel creation (avoids timing crash)
  - guard onGroupByChanged against null model
- EmailTreeModel.buildThreads: single-message 'threads' now shown as plain
  mail rows under root (no expander/nesting), matching Wino/Outlook behavior;
  only real 2+ message conversations render as collapsible thread groups

src/ui/maillistview.cpp

@@ -149,6 +149,8 @@ void MailListView::setupUI()
     );
     connect(m_groupByCombo, QOverload<int>::of(&QComboBox::currentIndexChanged),
             this, &MailListView::onGroupByChanged);
+    // Default to conversation grouping (Wino-style), index 2 = GroupByThread
+    m_groupByCombo->setCurrentIndex(2);
     filterLayout->addWidget(m_groupByCombo);
 
     layout->addWidget(filterBar);
@@ -244,6 +246,8 @@ void MailListView::setupUI()
     );
 
     m_treeModel = new EmailTreeModel(this);
+    // Wino-style default: group inbox by conversation (thread)
+    m_treeModel->setGroupMode(EmailTreeModel::GroupByThread);
     m_proxyModel = new QSortFilterProxyModel(this);
     m_proxyModel->setSourceModel(m_treeModel);
     m_proxyModel->setFilterCaseSensitivity(Qt::CaseInsensitive);
@@ -528,6 +532,7 @@ void MailListView::onFilterChanged()
 
 void MailListView::onGroupByChanged(int index)
 {
+    if (!m_treeModel) return;
     switch (index) {
     case 0: // Sin agrupación
         m_treeModel->setGroupMode(EmailTreeModel::NoGrouping);
@@ -542,7 +547,7 @@ void MailListView::onGroupByChanged(int index)
         m_treeModel->setGroupMode(EmailTreeModel::GroupBySender);
         break;
     }
-    m_treeView->expandAll();
+    if (m_treeView) m_treeView->expandAll();
 }
 
 void MailListView::onDisplayModeChanged(int index)

src/ui/models/EmailTreeModel.cpp

@@ -465,6 +465,16 @@ void EmailTreeModel::buildThreads()
             return dateA < dateB; // Oldest first
         });
 
+        if (sortedIndices.size() == 1) {
+            // A single-message "thread" isn't a real conversation — add it as a
+            // plain Mail row directly under root (no expander, no nesting).
+            TreeItem* mailItem = new TreeItem(TreeItem::Mail, m_rootItem);
+            mailItem->mailIndex = sortedIndices.first();
+            mailItem->row = m_rootItem->children.size();
+            m_rootItem->children.append(mailItem);
+            continue;
+        }
+
         TreeItem* threadItem = new TreeItem(TreeItem::Thread, m_rootItem);
         threadItem->threadId = threadId;
         threadItem->groupIndex = threadIdx++;