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/db/dao/mailitemdao.cpp

@@ -394,3 +394,19 @@ QVector<StoredAttachmentRecord> MailItemDao::attachmentsForMail(qint64 mailItemI
     }
     return result;
 }
+
+int MailItemDao::countUnreadByFolderId(int folderId)
+{
+    QSqlDatabase db = DatabaseManager::instance().database();
+    QSqlQuery query(db);
+    query.prepare("SELECT COUNT(*) FROM MailCopy WHERE folderId = :folderId AND read = 0");
+    query.bindValue(":folderId", folderId);
+    if (!query.exec()) {
+        qWarning() << "Failed to count unread mails:" << query.lastError().text();
+        return 0;
+    }
+    if (query.next()) {
+        return query.value(0).toInt();
+    }
+    return 0;
+}

src/db/dao/mailitemdao.h

@@ -32,4 +32,5 @@ public:
     static bool removeByUid(int folderId, qint64 uid);
     static bool replaceAttachments(qint64 mailItemId, const QVector<StoredAttachmentRecord>& attachments);
     static QVector<StoredAttachmentRecord> attachmentsForMail(qint64 mailItemId);
+    static int countUnreadByFolderId(int folderId);
 };