feat: email grouping by date + stability fixes

- EmailTreeModel: hierarchical model for grouped email view (QTreeView)
- DateGroupProxyModel: proxy model for date-based grouping logic
- MailListView: migrated from QTableView to QTreeView with grouping support
- ReaderView: subject frame with shadow, avatar moved to header, body inside header
- MainWindow: frameless with rounded corners, 1px border, edge resize, no status bar

- Stability fixes for grouping mode switch:
  * Proper tree cleanup (no double-delete of root)
  * Bounds checking on all array accesses
  * Null pointer checks in index/parent/data methods
  * Date validity checks before grouping
  * Index validation before accessing m_emails

src/ui/models/EmailTreeModel.h

@@ -0,0 +1,78 @@
+#pragma once
+
+#include <QAbstractItemModel>
+#include <QModelIndex>
+#include <QVariant>
+#include <QVector>
+#include <QDateTime>
+#include <QDate>
+#include "core/mailitem.h"
+
+class EmailTreeModel : public QAbstractItemModel
+{
+    Q_OBJECT
+
+public:
+    enum Columns {
+        ColSubject = 0,
+        ColSender = 1,
+        ColDate = 2,
+        ColCount = 3
+    };
+
+    enum Roles {
+        MailIdRole = Qt::UserRole + 1,
+        IsGroupRole = Qt::UserRole + 2,
+        GroupNameRole = Qt::UserRole + 3,
+        GroupIndexRole = Qt::UserRole + 4,
+        MailIndexRole = Qt::UserRole + 5
+    };
+
+    explicit EmailTreeModel(QObject *parent = nullptr);
+    ~EmailTreeModel() override;
+
+    // Basic model interface
+    QModelIndex index(int row, int column, const QModelIndex &parent = QModelIndex()) const override;
+    QModelIndex parent(const QModelIndex &child) const override;
+    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;
+    Qt::ItemFlags flags(const QModelIndex &index) const override;
+
+    // Email data management
+    void setEmails(const QVector<MailItem> &emails);
+    void clear();
+
+    // Grouping
+    enum GroupMode {
+        NoGrouping = 0,
+        GroupByDate = 1
+    };
+    void setGroupMode(GroupMode mode);
+    GroupMode groupMode() const { return m_groupMode; }
+
+private:
+    struct TreeItem {
+        enum Type { Group, Mail } type;
+        QString groupName;          // for Group type
+        int groupIndex = -1;        // for Group type
+        int mailIndex = -1;         // for Mail type (index in m_emails)
+        QVector<TreeItem*> children;
+        TreeItem* parent = nullptr;
+        int row = -1;
+
+        TreeItem(Type t, TreeItem* p = nullptr) : type(t), parent(p) {}
+        ~TreeItem() { qDeleteAll(children); }
+    };
+
+    GroupMode m_groupMode = NoGrouping;
+    QVector<MailItem> m_emails;
+    TreeItem* m_rootItem = nullptr;
+
+    void setupModelData();
+    void buildGroups();
+    QString groupNameForDate(const QDateTime &date) const;
+    int groupIndexForDate(const QDateTime &date) const;
+    void deleteTree(TreeItem* item);
+};