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)

src/ui/mainmainwindow.h

@@ -74,6 +74,12 @@ private slots:
     void showRulesManager();
     void showTemplatesManager();
 
+    // MailListView action handlers (from compact delegate)
+    void onFlagRequested(int mailId);
+    void onDeleteRequested(int mailId);
+    void onCategoryRequested(int mailId);
+    void onMoreRequested(int mailId, const QPoint &globalPos);
+
     // Slots for embedded compose view
     void onEmbeddedSendRequested(const QString &to, const QString &cc, const QString &bcc,
                                const QString &subject, const QString &body,