Add macOS Cocoa port and CEF helper support

Introduce a macOS Cocoa-based UI and CEF helper subprocess support. CMake: enable OBJCXX on Apple, treat mac sources as .mm, set -fobjc-arc, link Cocoa frameworks, and generate helper app targets using a mac Info.plist template; keep libcef logical target off macOS. Implementation: add Objective-C++ implementations for browser_host and nebula_window, convert startup to ObjC++ (prepare NSApplication), add process_helper_mac (CEF helper entry) and load CEF library from main/main_bigpicture on mac. Tooling/docs: add .clangd fallback flags, compile_commands symlink helper, update cross-platform docs for mac status. Misc: add menu-popup UI page, define UNREFERENCED_PARAMETER in platform/types.h, small code cleanups (use (void)layout, include platform/types.h) and update .gitignore.

app/main_bigpicture.cpp

@@ -3,7 +3,11 @@
 
 #if defined(_WIN32)
 #include <windows.h>
+#elif defined(__APPLE__)
+#include "include/wrapper/cef_library_loader.h"
+#endif
 
+#if defined(_WIN32)
 int APIENTRY wWinMain(HINSTANCE instance,
                       HINSTANCE previous_instance,
                       LPWSTR command_line,
@@ -16,6 +20,13 @@ int APIENTRY wWinMain(HINSTANCE instance,
 }
 #else
 int main(int argc, char* argv[]) {
+#if defined(__APPLE__)
+    CefScopedLibraryLoader library_loader;
+    if (!library_loader.LoadInMain()) {
+        return 1;
+    }
+#endif
+
     const nebula::platform::AppStartup startup{argc, argv};
     return nebula::app::RunNebula(startup, {nebula::app::AppMode::BigPicture});
 }