Fix Tailwind race condition in fullstack serve mode (#4433)

Skip Tailwind watcher in fullstack mode to prevent race conditions between
the watcher and prebuild() both trying to install/run Tailwind simultaneously.

In fullstack mode, the client build's prebuild() properly handles Tailwind
generation, making the watcher redundant and problematic.

Fixes "Text file busy" error when running:
dx serve --fullstack true --platform web

Fixes #4431

packages/cli/src/serve/runner.rs

@@ -166,11 +166,18 @@ impl AppServer {
         let client = AppBuilder::new(&client)?;
         let server = server.map(|server| AppBuilder::new(&server)).transpose()?;
 
-        let tw_watcher = TailwindCli::serve(
-            client.build.package_manifest_dir(),
-            client.build.config.application.tailwind_input.clone(),
-            client.build.config.application.tailwind_output.clone(),
-        );
+        // Only start Tailwind watcher for client builds that serve assets (not server builds or fullstack mode)
+        // In fullstack mode, the client build's prebuild() handles Tailwind generation to avoid race conditions
+        let tw_watcher = if client.build.platform != Platform::Server && !fullstack {
+            TailwindCli::serve(
+                client.build.package_manifest_dir(),
+                client.build.config.application.tailwind_input.clone(),
+                client.build.config.application.tailwind_output.clone(),
+            )
+        } else {
+            // Return a dummy task that immediately completes for server builds or fullstack mode
+            tokio::spawn(async { Ok(()) })
+        };
 
         _ = client.build.start_simulators().await;