fix: use v.Stdout in isTerminal() instead of os.Stdout.Fd()

Convert isTerminal() from a standalone function to a method on Vaultik
that checks whether v.Stdout implements Fd() (i.e. is an *os.File).
Falls back to non-terminal for non-file writers (e.g. in tests).

Eliminates all direct os.Stdout/os.Stderr references from restore.go.

internal/vaultik/restore.go

@@ -123,7 +123,7 @@ func (v *Vaultik) Restore(opts *RestoreOptions) error {
 
 	// Create progress bar if output is a terminal
 	var bar *progressbar.ProgressBar
-	if isTerminal() {
+	if v.isTerminal() {
 		bar = progressbar.NewOptions64(
 			totalBytesExpected,
 			progressbar.OptionSetDescription("Restoring"),
@@ -573,7 +573,7 @@ func (v *Vaultik) verifyRestoredFiles(
 
 	// Create progress bar if output is a terminal
 	var bar *progressbar.ProgressBar
-	if isTerminal() {
+	if v.isTerminal() {
 		bar = progressbar.NewOptions64(
 			totalBytes,
 			progressbar.OptionSetDescription("Verifying"),
@@ -681,7 +681,16 @@ func (v *Vaultik) verifyFile(
 	return bytesVerified, nil
 }
 
-// isTerminal returns true if stdout is a terminal
-func isTerminal() bool {
-	return term.IsTerminal(int(os.Stdout.Fd()))
+// isTerminal returns true if stdout is a terminal.
+// It checks whether v.Stdout implements Fd() (i.e. is an *os.File),
+// and falls back to false for non-file writers (e.g. in tests).
+func (v *Vaultik) isTerminal() bool {
+	type fder interface {
+		Fd() uintptr
+	}
+	f, ok := v.Stdout.(fder)
+	if !ok {
+		return false
+	}
+	return term.IsTerminal(int(f.Fd()))
 }