fix: skip LD_PRELOAD on macOS for studio containers

On macOS, when creating studio containers, skip setting LD_PRELOAD
environment variable because:
1. macOS host doesn't have GPU libraries (GPU is remote)
2. Attempting to load non-existent libraries causes ld.so errors in the
   Linux container:
   "ERROR: ld.so: object '/opt/gpugo/libs/libcuda.so' from
    /etc/ld.so.preload cannot be preloaded (cannot open shared object file)"

This fix checks the host platform using IsDarwin() and only sets
LD_PRELOAD when not on macOS.

Other platform-related fixes already present in codebase:
- Docker containers have default "sleep infinity" command (backend_docker.go:294)
- Default platform is "linux/amd64" for studio containers (studio.go:477)
- No container runtime error messages with helpful hints (manager.go:120-148)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: andystime

internal/studio/env.go

@@ -311,17 +311,20 @@ func SetupGPUEnv(paths *platform.Paths, config *GPUEnvConfig) (*GPUEnvResult, er
 		// This ensures only user shells get the preload, not system daemons like sshd
 
 		// Add LD_PRELOAD as environment variable that will be written to /etc/environment
-		ldPreloadContent := generateLDPreloadContent(config.Vendor, libsPath, config.IsContainer)
-		// Extract library paths from preload content (skip comment lines)
-		var preloadPaths []string
-		for _, line := range strings.Split(ldPreloadContent, "\n") {
-			line = strings.TrimSpace(line)
-			if line != "" && !strings.HasPrefix(line, "#") {
-				preloadPaths = append(preloadPaths, line)
+		// Skip on macOS host as GPU libraries are not available locally (GPU is remote)
+		if !IsDarwin() {
+			ldPreloadContent := generateLDPreloadContent(config.Vendor, libsPath, config.IsContainer)
+			// Extract library paths from preload content (skip comment lines)
+			var preloadPaths []string
+			for _, line := range strings.Split(ldPreloadContent, "\n") {
+				line = strings.TrimSpace(line)
+				if line != "" && !strings.HasPrefix(line, "#") {
+					preloadPaths = append(preloadPaths, line)
+				}
+			}
+			if len(preloadPaths) > 0 {
+				result.EnvVars["LD_PRELOAD"] = strings.Join(preloadPaths, ":")
 			}
-		}
-		if len(preloadPaths) > 0 {
-			result.EnvVars["LD_PRELOAD"] = strings.Join(preloadPaths, ":")
 		}
 
 		// Update env vars for container paths