feat: responsive compact mail view + folder unread badges

- CompactMailDelegate: card-based view for narrow panels (<420px)
  * Avatar with hash-based color, sender + date (relative format)
  * Subject line with attachment icon
  * Action buttons: flag, delete, category, more (context menu)
  * Unread highlight with light blue background + bold sender
- MailListView: automatic switch between tree/table (wide) and list/cards (narrow)
  * QStackedWidget with QTreeView + QListView sharing same proxy model
  * resizeEvent threshold at 420px
  * Signals forwarded to MainMainWindow for actions
- FolderTreeDelegate: custom paint for folder tree
  * Unread count badge (blue pill) right-aligned on folder nodes
  * Account nodes bold, folder nodes normal weight
  * Proper indentation per depth level
- MailItemDao::countUnreadByFolderId() for real-time unread counts
- MainMainWindow handlers for compact actions (flag, delete, category, more menu)

CMakeLists.txt

@@ -128,6 +128,10 @@ set(SRC_FILES
     src/ui/templatesmanagerdialog.h
     src/ui/signaturemanagerdialog.cpp
     src/ui/signaturemanagerdialog.h
+    src/ui/delegates/CompactMailDelegate.cpp
+    src/ui/delegates/CompactMailDelegate.h
+    src/ui/delegates/FolderTreeDelegate.cpp
+    src/ui/delegates/FolderTreeDelegate.h
     src/syncscheduler.cpp
     thirdparty/tags/src/tags_line_edit.cpp
     thirdparty/tags/src/tags_edit.cpp