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/bitcoin.h

@@ -96,16 +96,16 @@ protected:
 
 private:
     std::optional<InitExecutor> m_executor;
-    OptionsModel *optionsModel;
-    ClientModel *clientModel;
-    BitcoinGUI *window;
-    QTimer *pollShutdownTimer;
+    OptionsModel* optionsModel{nullptr};
+    ClientModel* clientModel{nullptr};
+    BitcoinGUI* window{nullptr};
+    QTimer* pollShutdownTimer{nullptr};
 #ifdef ENABLE_WALLET
     PaymentServer* paymentServer{nullptr};
     WalletController* m_wallet_controller{nullptr};
 #endif
-    int returnValue;
-    const PlatformStyle *platformStyle;
+    int returnValue{0};
+    const PlatformStyle* platformStyle{nullptr};
     std::unique_ptr<QWidget> shutdownWindow;
     SplashScreen* m_splash = nullptr;
     std::unique_ptr<interfaces::Node> m_node;