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/mailservice.cpp

@@ -503,4 +503,4 @@ void MailService::markAsRead(const QString &mailItemId, bool read)
     if (MailItemDao::update(item))
         emit mailReadStateChanged(mailItemId, read);
 }
-
+MailService::~MailService() = default;