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/EmailListModel.h

@@ -53,6 +53,9 @@ public:
     void setShowFlaggedOnly(bool show);
     void setShowHasAttachments(bool show);
 
+    // Get all emails currently in the model (after filtering)
+    const QVector<MailItem>& emails() const { return m_emails; }
+
 private:
     QVector<MailItem> m_emails;
     int m_folderId{-1}; // -1 means all folders