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.

.clangd

@@ -0,0 +1,17 @@
+# clangd configuration for Nebula Browser
+#
+# Full diagnostics require:
+#   1. CEF binary distribution unpacked to thirdparty/cef/
+#   2. Running: cmake -B build -DCMAKE_EXPORT_COMPILE_COMMANDS=ON
+#   3. Symlink or copy: ln -s build/compile_commands.json .
+#
+# Without compile_commands.json, clangd uses fallback flags below.
+
+CompileFlags:
+  Add:
+    - -std=c++20
+    - -xobjective-c++
+    - -Isrc
+    - -Ithirdparty/cef
+    - -Ithirdparty/cef/include
+    - -Wno-c++17-extensions