fix: SIGSEGV on exit - double-free in EmailTreeModel::deleteTree

deleteTree did qDeleteAll(children) AND delete item, but ~TreeItem already
frees children recursively (qDeleteAll(children)). The children were freed
twice -> heap-use-after-free -> SIGSEGV exactly on app teardown, only
visible when mail threads had real tree children (empty DB masked it).

Reproduced under ASAN with seeded thread data: old code = heap-use-after-free
exit 1; fixed code = clean exit 0. Reproducible, not guessed.

src/ui/models/EmailTreeModel.cpp

@@ -20,7 +20,9 @@ EmailTreeModel::~EmailTreeModel()
 void EmailTreeModel::deleteTree(TreeItem* item)
 {
     if (!item) return;
-    qDeleteAll(item->children);
+    // TreeItem::~TreeItem already deletes its children recursively (qDeleteAll(children)).
+    // Only delete the node itself — calling qDeleteAll(children) here first would
+    // double-free (the dtor frees them again) and SIGSEGV on teardown in debug builds.
     delete item;
 }