feat: Pagination/virtualization (fetchMore) for EmailListModel

- EmailListModel: QAbstractListModel with canFetchMore/fetchMore
  * Batch loading (default 50 emails per fetch)
  * Total count query for scrollbar
  * TotalCountChanged signal
  * Paginated DB queries: findAllPaginated, findByFolderIdPaginated
  * Count queries: countAll, countByFolderId

- MailItemDao: new paginated methods
  * findAllPaginated(offset, limit, filters...)
  * findByFolderIdPaginated(folderId, offset, limit, filters...)
  * countAll(filters...), countByFolderId(folderId, filters...)

- MailListView: scroll-triggered fetchMore
  * Connects verticalScrollBar valueChanged on both TreeView and ListView
  * Triggers fetchMore at 90% scroll threshold
  * onRowsInserted updates EmailTreeModel when new emails arrive
  * Works in both normal (tree) and compact (list) views

src/ui/models/EmailListModel.h

@@ -6,7 +6,7 @@
 #include <QByteArray>
 #include "../db/dao/mailitemdao.h"
 
-class EmailListModel : public QAbstractTableModel
+class EmailListModel : public QAbstractListModel
 {
     Q_OBJECT
 public:
@@ -25,22 +25,19 @@ public:
         FileIdRole,
         SizeRole,
         MessageIdRole,
-        SenderInitialRole // computed from sender
-    };
-
-    // Column indices (match role order for simplicity)
-    enum Columns {
-        ColSubject = 0,
-        ColSender = 1,
-        ColDate = 2,
-        ColCount = 3 // number of visible columns
+        SenderInitialRole, // computed from sender
+        ThreadIdRole,
+        InReplyToRole,
+        ReferencesRole,
+        IsPinnedRole
     };
 
+    // QAbstractListModel interface
     int rowCount(const QModelIndex &parent = QModelIndex()) const override;
-    int columnCount(const QModelIndex &parent = QModelIndex()) const override;
     QVariant data(const QModelIndex &index, int role = Qt::DisplayRole) const override;
-    QVariant headerData(int section, Qt::Orientation orientation, int role = Qt::DisplayRole) const override;
     QHash<int, QByteArray> roleNames() const override;
+    bool canFetchMore(const QModelIndex &parent) const override;
+    void fetchMore(const QModelIndex &parent) override;
 
     // Set the folderId to filter emails; -1 means all folders
     void setFolderId(int folderId);
@@ -52,10 +49,18 @@ public:
     void setShowUnreadOnly(bool show);
     void setShowFlaggedOnly(bool show);
     void setShowHasAttachments(bool show);
+    // Set batch size for pagination
+    void setBatchSize(int size) { m_batchSize = size; }
 
     // Get all emails currently in the model (after filtering)
     const QVector<MailItem>& emails() const { return m_emails; }
 
+    // Total count of emails matching filters (including unloaded)
+    int totalCount() const { return m_totalCount; }
+
+signals:
+    void totalCountChanged(int count);
+
 private:
     QVector<MailItem> m_emails;
     int m_folderId{-1}; // -1 means all folders
@@ -63,5 +68,8 @@ private:
     bool m_unreadOnly{false};
     bool m_flaggedOnly{false};
     bool m_hasAttachments{false};
+    int m_batchSize{50}; // emails per fetch
+    int m_loadedCount{0}; // how many emails currently loaded
+    int m_totalCount{0}; // total matching emails in DB
 };
 #endif // EMAILLISTMODEL_H