Support codex app on macOS (Intel) and Windows (#18500)

## Summary

`codex app` should be a platform-aware entry point for opening Codex
Desktop or helping users install it. Before this change, the command
only existed on macOS and its default installer URL always pointed at
the Apple Silicon DMG, which sent Intel Mac users to the wrong build.

This updates the macOS path to choose the Apple Silicon or Intel DMG
based on the detected processor, while keeping `--download-url` as an
advanced override. It also enables `codex app` on Windows, where the CLI
opens an installed Codex Desktop app when available and otherwise opens
the Windows installer URL.

---------

Co-authored-by: Felipe Coury <felipe.coury@openai.com>

codex-rs/cli/src/main.rs

@@ -34,9 +34,9 @@ use std::io::IsTerminal;
 use std::path::PathBuf;
 use supports_color::Stream;
 
-#[cfg(target_os = "macos")]
+#[cfg(any(target_os = "macos", target_os = "windows"))]
 mod app_cmd;
-#[cfg(target_os = "macos")]
+#[cfg(any(target_os = "macos", target_os = "windows"))]
 mod desktop_app;
 mod marketplace_cmd;
 mod mcp_cmd;
@@ -120,8 +120,8 @@ enum Subcommand {
     /// [experimental] Run the app server or related tooling.
     AppServer(AppServerCommand),
 
-    /// Launch the Codex desktop app (downloads the macOS installer if missing).
-    #[cfg(target_os = "macos")]
+    /// Launch the Codex desktop app (opens the app installer if missing).
+    #[cfg(any(target_os = "macos", target_os = "windows"))]
     App(app_cmd::AppCommand),
 
     /// Generate shell completion scripts.
@@ -808,7 +808,7 @@ async fn cli_main(arg0_paths: Arg0DispatchPaths) -> anyhow::Result<()> {
                 }
             }
         }
-        #[cfg(target_os = "macos")]
+        #[cfg(any(target_os = "macos", target_os = "windows"))]
         Some(Subcommand::App(app_cli)) => {
             reject_remote_mode_for_subcommand(
                 root_remote.as_deref(),