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/readerview.cpp

@@ -903,3 +903,4 @@ void ReaderView::onDeleteClicked() {
 void ReaderView::onDetachClicked() {
     emit detachRequested();
 }
+ReaderView::~ReaderView() = default;