Merge Profiles and Session windows into a single window

ProfilesWindow and SessionWindow were two independent top-level windows,
coordinated through a QPointer and manual show/create-or-reuse logic, with
duplicated Help menus and a path where Quit didn't actually quit if a
SessionWindow happened to be open. ProfilesWindow is now an embedded QWidget
shown as a permanent, unclosable "Profiles" tab inside SessionWindow's tab
widget; SessionWindow is the app's sole top-level window. Double-clicking a
profile opens a session tab in the same window instead of finding-or-creating
a second one, and closing all session tabs falls back to the Profiles tab
rather than closing the app.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/main.cpp

@@ -1,5 +1,5 @@
 #include "app_icon.h"
-#include "profiles_window.h"
+#include "session_window.h"
 
 #include <QApplication>
 
@@ -12,7 +12,7 @@ int main(int argc, char* argv[])
     app.setApplicationName(QStringLiteral("OrbitHub"));
     app.setWindowIcon(createOrbitHubAppIcon());
 
-    ProfilesWindow window;
+    SessionWindow window;
     window.show();
 
     return app.exec();