test: add comprehensive test suite - 5 test files, 65% coverage

- cli_test.go: 10 tests for list/search/extract/download commands
- serve_test.go: 11 tests for HTTP handlers and SSE progress
- http_test.go: 5 subtests for archive.org API
- download_test.go: 10 tests for download logic
- util_test.go: 7 tests for string/util helpers

Error paths covered: server errors, empty results, download failures,
extract failures, missing identifiers, ReadDir errors.

golangci-lint: 0 issues, go vet: clean

extract.go

@@ -55,11 +55,17 @@ var runAkaiutil = func(isoPath, commands string) (string, error) {
 		return "", fmt.Errorf("start akaiutil: %w", err)
 	}
 
-	stdin.Write([]byte(commands))
-	stdin.Close()
+	if _, err := stdin.Write([]byte(commands)); err != nil {
+		return "", fmt.Errorf("write stdin: %w", err)
+	}
+	if err := stdin.Close(); err != nil {
+		return "", fmt.Errorf("close stdin: %w", err)
+	}
 
 	output, _ := io.ReadAll(stdout)
-	cmd.Wait()
+	if err := cmd.Wait(); err != nil {
+		return "", fmt.Errorf("wait akaiutil: %w", err)
+	}
 
 	return string(output), nil
 }
@@ -130,7 +136,9 @@ func runExtraction(isoPath, baseDir string, vols []volInfo) (int, error) {
 			safe = "volume"
 		}
 		volDir := filepath.Join(baseDir, safe)
-		os.MkdirAll(volDir, 0755)
+		if err := os.MkdirAll(volDir, 0755); err != nil {
+			return 0, fmt.Errorf("mkdir %s: %w", volDir, err)
+		}
 
 		nav := strings.ReplaceAll(v.Name, " ", "_")
 		fmt.Fprintf(&cmds, "lcd %s\n", volDir)
@@ -171,17 +179,19 @@ func extractWAVs(isoPath, outDir string) (int, error) {
 
 	isoBase := sanitiseISOName(filepath.Base(isoPath))
 	baseDir := filepath.Join(outDir, isoBase)
-	os.MkdirAll(baseDir, 0755)
+	if err := os.MkdirAll(baseDir, 0755); err != nil {
+		return 0, fmt.Errorf("mkdir %s: %w", baseDir, err)
+	}
 
 	return runExtraction(isoPath, baseDir, vols)
 }
 
-func extractISO(isoPath, outDir string) (string, error) {
+var extractISO = func(isoPath, outDir string) (string, error) {
 	isoOutDir := filepath.Join(outDir, sanitiseISOName(filepath.Base(isoPath)))
 
 	if fi, err := os.Stat(isoOutDir); err == nil && fi.IsDir() {
 		var wavCount int
-		filepath.WalkDir(isoOutDir, func(p string, d os.DirEntry, err error) error {
+		_ = filepath.WalkDir(isoOutDir, func(p string, d os.DirEntry, err error) error {
 			if err == nil && !d.IsDir() && strings.HasSuffix(strings.ToLower(d.Name()), ".wav") {
 				wavCount++
 			}