Windows build support: conditional macOS app menu, devtools feature

- Gate OpenPencil app submenu (About/Services/Hide/Quit) behind
  #[cfg(target_os = "macos")] — Windows/Linux don't have it
- Enable tauri "devtools" feature in Cargo.toml (required for
  open_devtools/close_devtools on non-macOS)

desktop/Cargo.toml

@@ -18,7 +18,7 @@ crate-type = ["staticlib", "cdylib", "rlib"]
 tauri-build = { version = "2", features = [] }
 
 [dependencies]
-tauri = { version = "2", features = [] }
+tauri = { version = "2", features = ["devtools"] }
 tauri-plugin-opener = "2"
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"

desktop/src/lib.rs

@@ -22,6 +22,7 @@ pub fn run() {
             let _ = app.emit("menu-event", event.id().0.as_str());
         })
         .setup(|app| {
+            #[cfg(target_os = "macos")]
             let app_menu = SubmenuBuilder::new(app, "OpenPencil")
                 .item(&PredefinedMenuItem::about(app, Some("About OpenPencil"), None)?)
                 .separator()
@@ -223,9 +224,13 @@ pub fn run() {
                 )
                 .build()?;
 
-            let menu = MenuBuilder::new(app)
+            let mut builder = MenuBuilder::new(app);
+            #[cfg(target_os = "macos")]
+            {
+                builder = builder.item(&app_menu);
+            }
+            let menu = builder
                 .items(&[
-                    &app_menu,
                     &file_menu,
                     &edit_menu,
                     &view_menu,