fix: resolve back step, audio labels, progress parsing, file size pre-check, and stale job cleanup

- Fix handleBackStep to redirect to download prompt when step state is nil
  (previously silent no-op after container selection cleared the state)
- Improve formatLabel for audio-only entries without bitrate: use ID + extension
  instead of generic 'audio', fixing duplicate entries in format list
- Add yt-dlp --progress-template for reliable JSON progress parsing across all
  sites (fixes 0.0% progress on sites like xnxx.com where [download] lines
  are absent); keep regex parser as fallback for older yt-dlp versions
- Add file size pre-check in handleContainerSelection before download starts
  (previously checked only after download completed, wasting bandwidth)
- Clear stale active jobs and step state on /start for a clean slate

internal/dl/progress.go

@@ -12,6 +12,7 @@ import (
 )
 
 var progressRE = regexp.MustCompile(`\[\w+\]\s+([\d.]+)%\s+of\s+~?([\d.]+)(\w+)`)
+var progressTemplateRE = regexp.MustCompile(`^DLPROG\|([\d.]+)\|(.*?)\|(.*)$`)
 
 type progressUpdate struct {
 	pct   float64
@@ -19,11 +20,39 @@ type progressUpdate struct {
 	eta   string
 }
 
+// parseProgressTemplate parses a DLPROG-prefixed line from yt-dlp's --progress-template.
+func parseProgressTemplate(line string) *progressUpdate {
+	m := progressTemplateRE.FindStringSubmatch(line)
+	if m == nil {
+		return nil
+	}
+	pct, err := strconv.ParseFloat(m[1], 64)
+	if err != nil {
+		return nil
+	}
+	u := &progressUpdate{pct: pct}
+	speed := strings.TrimSpace(m[2])
+	eta := strings.TrimSpace(m[3])
+	if speed != "" && !strings.EqualFold(speed, "unknown") && speed != "N/A" {
+		u.speed = speed
+	}
+	if eta != "" && !strings.EqualFold(eta, "unknown") && eta != "N/A" {
+		u.eta = eta
+	}
+	return u
+}
+
 // parseProgressLine parses a single yt-dlp stderr progress line.
 // Returns nil if the line doesn't contain progress info.
 func parseProgressLine(line string) *progressUpdate {
 	line = strings.TrimSpace(line)
-	if line == "" || !strings.HasPrefix(line, "[download]") {
+	if line == "" {
+		return nil
+	}
+	if u := parseProgressTemplate(line); u != nil {
+		return u
+	}
+	if !strings.HasPrefix(line, "[download]") {
 		return nil
 	}
 	m := progressRE.FindStringSubmatch(line)