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/flutter/generated_plugin_registrant.h

@@ -0,0 +1,15 @@
+//
+//  Generated file. Do not edit.
+//
+
+// clang-format off
+
+#ifndef GENERATED_PLUGIN_REGISTRANT_
+#define GENERATED_PLUGIN_REGISTRANT_
+
+#include <flutter_linux/flutter_linux.h>
+
+// Registers Flutter plugins.
+void fl_register_plugins(FlPluginRegistry* registry);
+
+#endif  // GENERATED_PLUGIN_REGISTRANT_