fix: SIGSEGV at startup - uninitialized m_treeModel pointer

m_treeModel/m_proxyModel were declared without '= nullptr', so the
GroupByThread default applied during setupUI read garbage memory in debug
builds (QtCreator/MinGW) -> condition '!m_treeModel' was false against junk
and called setGroupMode on an invalid pointer -> SIGSEGV.

- Initialize both pointers to nullptr in header
- Apply combo default to index 2 only after m_treeModel is created (blockSignals)
- Remove the premature setCurrentIndex(2) that fired before model creation

src/ui/maillistview.cpp

@@ -149,8 +149,6 @@ 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);
@@ -248,6 +246,10 @@ void MailListView::setupUI()
     m_treeModel = new EmailTreeModel(this);
     // Wino-style default: group inbox by conversation (thread)
     m_treeModel->setGroupMode(EmailTreeModel::GroupByThread);
+    // Reflect the default in the group combo (model already set, block to avoid re-trigger)
+    m_groupByCombo->blockSignals(true);
+    m_groupByCombo->setCurrentIndex(2);  // "Agrupar por conversación"
+    m_groupByCombo->blockSignals(false);
     m_proxyModel = new QSortFilterProxyModel(this);
     m_proxyModel->setSourceModel(m_treeModel);
     m_proxyModel->setFilterCaseSensitivity(Qt::CaseInsensitive);