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/readerview.h

@@ -19,7 +19,7 @@ class ReaderView : public QWidget {
 
 public:
     explicit ReaderView(QWidget *parent = nullptr);
-    ~ReaderView() override = default;
+    ~ReaderView() override;
 
     void setMailItem(const MailItem* item);
     void setDarkMode(bool dark);