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

tests/unit/test_accountdao.pro

@@ -0,0 +1,7 @@
+QT += core testlib sql
+CONFIG += console c++17
+SOURCES += ../../src/db/dao/accountdao.cpp \
+           ../../src/db/databasemanager.cpp \
+           ../../src/core/models/account.cpp \
+           test_accountdao.cpp
+INCLUDEPATH += ../../src ../../src/services ../../src/db ../../src/db/dao ../../src/core ../../src/core/models