fix: address PR #24 review concerns

1. Replace os.Stderr with v.Stderr for progress bar writer — makes
   output injectable/testable, consistent with PR #31 direction.

2. Fix isTerminal() to accept io.Writer and check v.Stderr fd instead
   of os.Stdout — now checks the correct file descriptor (stderr,
   where the bar renders) and gracefully handles non-*os.File writers
   (returns false in tests).

3. Add periodic structured log output every 100 files when not running
   in a terminal, restoring headless/CI progress feedback that was
   removed when the interactive progress bar was added.

4. Apply same os.Stderr -> v.Stderr fix to the verify progress bar
   for consistency.

internal/vaultik/restore.go

@@ -126,24 +126,26 @@ func (v *Vaultik) Restore(opts *RestoreOptions) error {
 		humanize.Bytes(uint64(totalBytes)),
 	)
 
-	// Create progress bar if output is a terminal
+	// Create progress bar if stderr is a terminal
+	isTTY := isTerminal(v.Stderr)
 	var bar *progressbar.ProgressBar
-	if isTerminal() {
+	if isTTY {
 		bar = progressbar.NewOptions64(
 			totalBytes,
 			progressbar.OptionSetDescription("Restoring"),
-			progressbar.OptionSetWriter(os.Stderr),
+			progressbar.OptionSetWriter(v.Stderr),
 			progressbar.OptionShowBytes(true),
 			progressbar.OptionShowCount(),
 			progressbar.OptionSetWidth(40),
 			progressbar.OptionThrottle(100*time.Millisecond),
 			progressbar.OptionOnCompletion(func() {
-				fmt.Fprint(os.Stderr, "\n")
+				fmt.Fprint(v.Stderr, "\n")
 			}),
 			progressbar.OptionSetRenderBlankState(true),
 		)
 	}
 
+	filesProcessed := 0
 	for _, file := range files {
 		if v.ctx.Err() != nil {
 			return v.ctx.Err()
@@ -151,17 +153,33 @@ func (v *Vaultik) Restore(opts *RestoreOptions) error {
 
 		if err := v.restoreFile(v.ctx, repos, file, opts.TargetDir, identity, chunkToBlobMap, blobCache, result); err != nil {
 			log.Error("Failed to restore file", "path", file.Path, "error", err)
+			filesProcessed++
 			// Update progress bar even on failure
 			if bar != nil {
 				_ = bar.Add64(file.Size)
 			}
+			// Periodic structured log for non-terminal contexts (headless/CI)
+			if !isTTY && filesProcessed%100 == 0 {
+				log.Info("Restore progress",
+					"files", fmt.Sprintf("%d/%d", filesProcessed, len(files)),
+					"bytes_restored", humanize.Bytes(uint64(result.BytesRestored)),
+				)
+			}
 			continue
 		}
 
+		filesProcessed++
 		// Update progress bar
 		if bar != nil {
 			_ = bar.Add64(file.Size)
 		}
+		// Periodic structured log for non-terminal contexts (headless/CI)
+		if !isTTY && (filesProcessed%100 == 0 || filesProcessed == len(files)) {
+			log.Info("Restore progress",
+				"files", fmt.Sprintf("%d/%d", filesProcessed, len(files)),
+				"bytes_restored", humanize.Bytes(uint64(result.BytesRestored)),
+			)
+		}
 	}
 
 	if bar != nil {
@@ -555,7 +573,7 @@ func (v *Vaultik) verifyRestoredFiles(
 
 	// Create progress bar if output is a terminal
 	var bar *progressbar.ProgressBar
-	if isTerminal() {
+	if isTerminal(v.Stderr) {
 		bar = progressbar.NewOptions64(
 			totalBytes,
 			progressbar.OptionSetDescription("Verifying"),
@@ -663,7 +681,11 @@ func (v *Vaultik) verifyFile(
 	return bytesVerified, nil
 }
 
-// isTerminal returns true if stdout is a terminal
+// isTerminal returns true if the given writer is connected to a terminal.
-func isTerminal() bool {
+// Returns false if the writer does not expose a file descriptor (e.g. in tests).
-	return term.IsTerminal(int(os.Stdout.Fd()))
+func isTerminal(w io.Writer) bool {
+	if f, ok := w.(*os.File); ok {
+		return term.IsTerminal(int(f.Fd()))
+	}
+	return false
 }