Add data directory selection dialog on first run

Shows an intro dialog on first launch letting users choose where to store
blockchain data. Saves the choice in QSettings so it only appears once.
Styled to match the existing Triangles dark theme.

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

src/qt/triangles.cpp

@@ -8,6 +8,7 @@
 #include "guiutil.h"
 #include "guiconstants.h"
 
+#include "introdialog.h"
 #include "init.h"
 #include "ui_interface.h"
 #include "qtipcserver.h"
@@ -131,6 +132,10 @@ int main(int argc, char *argv[])
     // Command-line options take precedence:
     ParseParameters(argc, argv);
 
+    // Show data directory selection dialog on first run (unless -datadir was passed)
+    if (!IntroDialog::pickDataDirectory())
+        return 0;
+
     // ... then triangles.conf:
     if (!boost::filesystem::is_directory(GetDataDir(false)))
     {