fix: cross-compilation darwin — checkBinary déplacé dans clipboard_unix.go

- checkBinary migré de clipboard_linux.go (build tag linux) vers clipboard_unix.go (build tag !windows)
- Corrige l'erreur 'undefined: checkBinary' lors du cross-compile darwin
- Évite le warning unused sur Windows (la fonction n'est compilée que sur !windows)
- Validation: vet + lint + race + build + cross-compile (linux/windows/darwin × amd64/arm64) OK

internal/clipboard/clipboard_linux.go

@@ -70,12 +70,3 @@ func (c *linuxClipboard) Write(text string) error {
 	}
 	return nil
 }
-
-// checkBinary verifies that a command exists in $PATH.
-func checkBinary(name string) error {
-	_, err := exec.LookPath(name)
-	if err != nil {
-		return fmt.Errorf("%s: %w", name, err)
-	}
-	return nil
-}

internal/clipboard/clipboard_unix.go

@@ -0,0 +1,18 @@
+//go:build !windows
+
+package clipboard
+
+import (
+	"fmt"
+	"os/exec"
+)
+
+// checkBinary verifies that a command exists in $PATH.
+// Only used on Linux and macOS (not Windows, which uses Win32 API directly).
+func checkBinary(name string) error {
+	_, err := exec.LookPath(name)
+	if err != nil {
+		return fmt.Errorf("%s: %w", name, err)
+	}
+	return nil
+}