Merge bitcoin-core/gui#686: clang-tidy: Force checks for headers in src/qt

7b7cd11244 clang-tidy, qt: Force checks for headers in `src/qt` (Hennadii Stepanov)
69eacf2c5e clang-tidy, qt: Fix `modernize-use-default-member-init` in headers (Hennadii Stepanov)

Pull request description:

  This PR split from bitcoin/bitcoin#26705 and contains only changes in `src/qt`.

  Effectively, it fixes the clang-tidy's `modernize-use-default-member-init` errors, and forces clang-tidy checks for all headers in the `src/qt` directory.

ACKs for top commit:
  jarolrod:
    ACK 7b7cd11244

Tree-SHA512: 79525bb0f31ae7cad88c781e55091a21467c0485ddc1ed03ad62e051480fda3b3710619ea11af480437edba3c6e038f7c40edc6b373e3a37408c006d11b34686

src/qt/notificator.h

@@ -61,10 +61,10 @@ private:
         UserNotificationCenter      /**< Use the 10.8+ User Notification Center (Mac only) */
     };
     QString programName;
-    Mode mode;
+    Mode mode{None};
     QSystemTrayIcon *trayIcon;
 #ifdef USE_DBUS
-    QDBusInterface *interface;
+    QDBusInterface* interface{nullptr};
 
     void notifyDBus(Class cls, const QString &title, const QString &text, const QIcon &icon, int millisTimeout);
 #endif