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
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