fix: remove =default from all destructors in headers

- Removed '= default' from all class destructors in headers
- Destructors now declared without =default (defined in .cpp)
- Fixes 'definition of explicitly-defaulted destructor' error across all classes

src/ui/settingsview.h

@@ -28,7 +28,7 @@ class SettingsView : public QWidget {
 
 public:
     explicit SettingsView(QWidget *parent = nullptr);
-    ~SettingsView() override = default;
+    ~SettingsView() override;
 
     void setAccountService(AccountService *service);
     void setPreferencesService(PreferencesService *service);