feat: Drag & drop between folders

- MailListView: setDragEnabled, setAcceptDrops, setDropIndicatorShown, setDragDropMode(DragDrop)
- Custom MIME type: application/x-wino-mail-ids with QVector<int> mail IDs
- mousePressEvent + mouseMoveEvent for drag initiation (standard Qt drag distance)
- initDrag() creates QDrag with selected mail IDs
- dragEnterEvent, dragMoveEvent, dropEvent handlers
- dropEvent emits batchMoveRequested(mailIds, targetFolderId)
- Works in both tree view and compact list view

build/CMakeFiles/wino-mail-qt.dir/src/ui/maillistview.cpp.o.d

@@ -602,4 +602,8 @@ CMakeFiles/wino-mail-qt.dir/src/ui/maillistview.cpp.o: \
  /usr/include/x86_64-linux-gnu/qt6/QtWidgets/QToolButton \
  /usr/include/x86_64-linux-gnu/qt6/QtWidgets/qtoolbutton.h \
  /usr/include/x86_64-linux-gnu/qt6/QtWidgets/QMenu \
- /usr/include/x86_64-linux-gnu/qt6/QtWidgets/qmenu.h
+ /usr/include/x86_64-linux-gnu/qt6/QtWidgets/qmenu.h \
+ /usr/include/x86_64-linux-gnu/qt6/QtGui/QDrag \
+ /usr/include/x86_64-linux-gnu/qt6/QtGui/qdrag.h \
+ /usr/include/x86_64-linux-gnu/qt6/QtCore/QMimeData \
+ /usr/include/x86_64-linux-gnu/qt6/QtCore/qmimedata.h