Improve startup performance, shutdown reliability, and Qt5 compatibility

- Defer SecureMsgStart and ReacceptWalletTransactions to background thread
  so GUI appears faster during startup
- Add shutdown checks in wallet scan/reaccept to prevent hanging on exit
- Smart wallet rescan: start from wallet birthday instead of genesis block
- Raise default dbcache from 25 to 128 MB for better performance
- Replace deprecated QDesktopServices with QStandardPaths (Qt5 compat)
- Enforce Qt5+ requirement in build system, remove -fpermissive flag

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/qt/trianglesgui.cpp

@@ -64,7 +64,7 @@
 #include <QDateTime>
 #include <QMovie>
 #include <QFileDialog>
-#include <QDesktopServices>
+#include <QStandardPaths>
 #include <QTimer>
 #include <QDragEnterEvent>
 #if QT_VERSION < 0x050000
@@ -1368,7 +1368,7 @@ void TrianglesGUI::encryptWallet(bool status)
 
 void TrianglesGUI::backupWallet()
 {
-    QString saveDir = QDesktopServices::storageLocation(QDesktopServices::DocumentsLocation);
+    QString saveDir = QStandardPaths::writableLocation(QStandardPaths::DocumentsLocation);
     QString filename = QFileDialog::getSaveFileName(this, tr("Backup Wallet"), saveDir, tr("Wallet Data (*.dat)"));
     if(!filename.isEmpty()) {
         if(!walletModel->backupWallet(filename)) {