fix(build): limit stderr suppression to Darwin only

Conservative approach to avoid cross-platform build issues.
Only enable file descriptor manipulation on macOS where it's tested.

- stderr_darwin.go: macOS-only implementation with syscall.Dup2
- stderr_other.go: no-op for all other platforms (Linux, Windows, etc.)

This ensures GoReleaser can build for all platforms successfully.
SDK warnings may appear on non-macOS platforms but that's acceptable.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: jackspirou

cmd/starmap/cmd/auth/stderr_unix.go cmd/starmap/cmd/auth/stderr_darwin.gocmd/starmap/cmd/auth/stderr_unix.go renamed to cmd/starmap/cmd/auth/stderr_darwin.go

@@ -1,4 +1,4 @@
-//go:build (darwin || linux) && !js && !wasip1
+//go:build darwin
 
 package auth
 

cmd/starmap/cmd/auth/stderr_other.go

@@ -0,0 +1,9 @@
+//go:build !darwin
+
+package auth
+
+// suppressStderr on non-Darwin platforms just runs the function normally.
+// Stderr suppression is only implemented on macOS (Darwin).
+func suppressStderr(fn func() error) error {
+	return fn()
+}