feat: Focused/Other pivot for Inbox

- MailListView: new m_pivotCombo (Principal/Otros) visible only for Inbox folders
- MailListView::setFolderType(folderType) called from MainMainWindow::onFolderSelected()
- Folder::type() returns 'inbox', 'sent', 'drafts', 'trash', 'custom'
- onPivotChanged() placeholder for future AI-based filtering (Focused vs Other)
- UI hidden for non-Inbox folders

src/ui/maillistview.cpp

@@ -77,6 +77,22 @@ void MailListView::setupUI()
 
     filterLayout->addStretch();
 
+    // Pivot control (Focused/Other) - only shown for Inbox folders
+    m_pivotCombo = new QComboBox();
+    m_pivotCombo->addItem("Principal");  // Focused
+    m_pivotCombo->addItem("Otros");      // Other
+    m_pivotCombo->setFixedWidth(140);
+    m_pivotCombo->setStyleSheet(
+        "QComboBox { border: 1px solid #d1d1d6; border-radius: 4px; padding: 4px 8px; "
+        "background: white; min-height: 20px; }"
+        "QComboBox:hover { border-color: #bdbdbd; }"
+        "QComboBox:focus { border-color: #1976D2; }"
+    );
+    m_pivotCombo->setVisible(false); // Only visible for Inbox
+    connect(m_pivotCombo, QOverload<int>::of(&QComboBox::currentIndexChanged),
+            this, &MailListView::onPivotChanged);
+    filterLayout->addWidget(m_pivotCombo);
+
     // Display mode combo (Compact/Medium/Spacious)
     m_displayModeCombo = new QComboBox();
     m_displayModeCombo->addItem("Compacto");
@@ -581,4 +597,26 @@ void MailListView::onBatchDeleteClicked()
     if (m_selectedMailIds.isEmpty()) return;
     emit batchDeleteRequested(m_selectedMailIds);
     exitMultiSelectMode();
+}
+
+void MailListView::setFolderType(const QString& folderType)
+{
+    m_pivotCombo->setVisible(folderType == "inbox");
+}
+
+void MailListView::onPivotChanged(int index)
+{
+    // 0 = Focused (Principal), 1 = Other (Otros)
+    // TODO: Implement pivot filtering logic
+    // For now, just update the model filter
+    if (!m_sourceModel) return;
+    
+    if (index == 0) {
+        // Focused: important emails (could be based on sender frequency, AI, etc.)
+        // m_sourceModel->setShowFocusedOnly(true);
+    } else {
+        // Other: promotional, notifications, etc.
+        // m_sourceModel->setShowFocusedOnly(false);
+    }
+    refreshViews();
 }