fix: add missing destructor definitions in .cpp files

- Added destructor definitions (= default) to all classes that had them declared in header but not defined
- Fixes 'undefined reference to destructor' linker errors across 30+ classes
- Covers services, UI components, and synchronizers

src/ui/maillistview.cpp

@@ -768,4 +768,5 @@ void MailListView::mouseMoveEvent(QMouseEvent *event)
         return;
     
     initDrag();
-}
+}
+MailListView::~MailListView() = default;