Add Flutter platform stubs for iOS, web, Linux, macOS, Windows

Auto-generated by flutter create; not currently targeted but kept for
future portability.

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

linux/runner/main.cc

@@ -0,0 +1,6 @@
+#include "my_application.h"
+
+int main(int argc, char** argv) {
+  g_autoptr(MyApplication) app = my_application_new();
+  return g_application_run(G_APPLICATION(app), argc, argv);
+}