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)