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)

thirdparty/mailio/lib/pkgconfig/mailio.pc

@@ -0,0 +1,12 @@
+prefix=/usr/local
+exec_prefix=/usr/local
+libdir=lib
+sharedlibdir=lib
+includedir=include
+
+Name: mailio
+Description: Cross-platform MIME library for SMTP, POP3, and IMAP protocols.
+Version: 0.26.0
+Requires:
+Libs: -L${libdir} -L${sharedlibdir} -lmailio
+Cflags: -I${includedir}