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/signaturemanagerdialog.cpp

@@ -274,4 +274,5 @@ void SignatureManagerDialog::onSignatureSelected(QListWidgetItem* item)
     if (item) {
         m_selectedSignatureHtml = item->data(Qt::UserRole + 1).toString();
     }
-}
+}
+SignatureManagerDialog::~SignatureManagerDialog() = default;