feat: email grouping by date + stability fixes

- EmailTreeModel: hierarchical model for grouped email view (QTreeView)
- DateGroupProxyModel: proxy model for date-based grouping logic
- MailListView: migrated from QTableView to QTreeView with grouping support
- ReaderView: subject frame with shadow, avatar moved to header, body inside header
- MainWindow: frameless with rounded corners, 1px border, edge resize, no status bar

- Stability fixes for grouping mode switch:
  * Proper tree cleanup (no double-delete of root)
  * Bounds checking on all array accesses
  * Null pointer checks in index/parent/data methods
  * Date validity checks before grouping
  * Index validation before accessing m_emails

src/ui/mainmainwindow.cpp

@@ -385,7 +385,7 @@ void MainMainWindow::onFolderSelected(const QModelIndex &index)
         m_emailModel->refresh();
         // Clear selection and show placeholder
         m_currentMailId = -1;
-        m_mailListView->tableView()->clearSelection();
+        m_mailListView->treeView()->clearSelection();
         m_viewerStack->setCurrentIndex(0); // placeholder
     }
 }