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/services/synchronizer.cpp

@@ -33,4 +33,5 @@ bool Synchronizer::deleteMailItem(const QString &folderId, const QString &itemUi
     Q_UNUSED(folderId);
     Q_UNUSED(itemUid);
     return false;
-}
+}
+Synchronizer::~Synchronizer() = default;