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

CMakeLists.txt

@@ -97,6 +97,8 @@ set(SRC_FILES
     src/ui/accountsetupdialog.h
     src/ui/models/FolderListModel.cpp
     src/ui/models/EmailListModel.cpp
+    src/ui/models/EmailTreeModel.cpp
+    src/ui/models/DateGroupProxyModel.cpp
     src/ui/readerview.cpp
     src/ui/readerview.h
     src/ui/maillistview.cpp