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/.qmake.stash

@@ -0,0 +1,23 @@
+QMAKE_CXX.QT_COMPILER_STDCXX = 201703L
+QMAKE_CXX.QMAKE_GCC_MAJOR_VERSION = 15
+QMAKE_CXX.QMAKE_GCC_MINOR_VERSION = 2
+QMAKE_CXX.QMAKE_GCC_PATCH_VERSION = 0
+QMAKE_CXX.COMPILER_MACROS = \
+    QT_COMPILER_STDCXX \
+    QMAKE_GCC_MAJOR_VERSION \
+    QMAKE_GCC_MINOR_VERSION \
+    QMAKE_GCC_PATCH_VERSION
+QMAKE_CXX.INCDIRS = \
+    /usr/include/c++/15 \
+    /usr/include/x86_64-linux-gnu/c++/15 \
+    /usr/include/c++/15/backward \
+    /usr/lib/gcc/x86_64-linux-gnu/15/include \
+    /usr/local/include \
+    /usr/include/x86_64-linux-gnu \
+    /usr/include
+QMAKE_CXX.LIBDIRS = \
+    /usr/lib/gcc/x86_64-linux-gnu/15 \
+    /usr/lib/x86_64-linux-gnu \
+    /usr/lib \
+    /lib/x86_64-linux-gnu \
+    /lib