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/mainmainwindow.cpp

@@ -388,6 +388,8 @@ void MainMainWindow::onFolderSelected(const QModelIndex &index)
             Account* account = m_accountService->findAccountById(folder.accountId());
             if (account) {
                 m_mailService->fetchMails(QString::number(account->id()), QString::number(m_currentFolderId));
+                // Tell MailListView the folder type for Focused/Other pivot
+                m_mailListView->setFolderType(folder.type().toLower()); // "inbox", "sent", etc.
                 delete account;
             }
         }