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_test.go

@@ -100,6 +100,79 @@ func TestParseProgressLine(t *testing.T) {
 	}
 }
 
+func TestParseProgressTemplate(t *testing.T) {
+	tests := []struct {
+		line   string
+		pct    float64
+		speed  string
+		eta    string
+		hasRes bool
+	}{
+		{
+			line:   "DLPROG|45.2|2.34MiB/s|00:45",
+			pct:    45.2,
+			speed:  "2.34MiB/s",
+			eta:    "00:45",
+			hasRes: true,
+		},
+		{
+			line:   "DLPROG|100.0|5.00MiB/s|00:00",
+			pct:    100.0,
+			speed:  "5.00MiB/s",
+			eta:    "00:00",
+			hasRes: true,
+		},
+		{
+			line:   "DLPROG|0.0||",
+			pct:    0.0,
+			speed:  "",
+			eta:    "",
+			hasRes: true,
+		},
+		{
+			line:   "not a progress line",
+			hasRes: false,
+		},
+		{
+			line:   "",
+			hasRes: false,
+		},
+		{
+			line:   "[download] 45.2% of 123.45MiB at 2.34MiB/s ETA 00:45",
+			hasRes: false,
+		},
+		{
+			line:   "DLPROG|50.0|Unknown|Unknown",
+			pct:    50.0,
+			speed:  "",
+			eta:    "",
+			hasRes: true,
+		},
+	}
+	for _, tt := range tests {
+		got := parseProgressTemplate(tt.line)
+		if !tt.hasRes {
+			if got != nil {
+				t.Errorf("parseProgressTemplate(%q) = %+v, want nil", tt.line, got)
+			}
+			continue
+		}
+		if got == nil {
+			t.Errorf("parseProgressTemplate(%q) = nil, want result", tt.line)
+			continue
+		}
+		if math.Abs(got.pct-tt.pct) > 0.01 {
+			t.Errorf("parseProgressTemplate(%q).pct = %f, want %f", tt.line, got.pct, tt.pct)
+		}
+		if got.speed != tt.speed {
+			t.Errorf("parseProgressTemplate(%q).speed = %q, want %q", tt.line, got.speed, tt.speed)
+		}
+		if got.eta != tt.eta {
+			t.Errorf("parseProgressTemplate(%q).eta = %q, want %q", tt.line, got.eta, tt.eta)
+		}
+	}
+}
+
 func TestParseProgressLineEdgeCases(t *testing.T) {
 	tests := []struct {
 		line   string