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)
javier
4c4fec7a0a