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/mainmainwindow.cpp

@@ -1171,3 +1171,4 @@ void MainMainWindow::onOnlineSearchRequested(const QString& query)
     
     delete account;
 }
+MainMainWindow::~MainMainWindow() = default;