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/accountsetupdialog.cpp

@@ -687,4 +687,4 @@ void AccountSetupDialog::showError(const QString &message)
     m_btnCancel->setVisible(true);
     m_btnNext->setVisible(false);
 }
-
+AccountSetupDialog::~AccountSetupDialog() = default;