Switch dashboard to watcher-based multi-disk view, fix transcoding FPS display

- dashboard.html: remove standalone "Mounted Disk" input panel; show all disks
  from GET /api/disks (watcher), auto-refresh every 5s
- detect.go: use avg_frame_rate when r_frame_rate is unrealistic (>120 fps or 0),
  fixes MJPEG/mjpeg showing 90000fps
- transcoder.go: parse fps= from ffmpeg progress output and expose in Progress struct
- copier.go: update task message with real-time encoding fps (@ 45.3 fps),
  clear speed_bps/eta during transcoding to avoid showing stale copy speed

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/transcoder/transcoder.go

@@ -21,8 +21,13 @@ type Options struct {
 	SourceInfo VideoInfo
 }
 
-// Transcode запускает ffmpeg. progress вызывается с 0..1 по мере работы.
-func Transcode(ctx context.Context, opts Options, progress func(float64)) error {
+type Progress struct {
+	Pct        float64 // 0..1
+	EncodeFPS  float64 // текущая скорость кодирования, кадр/с
+}
+
+// Transcode запускает ffmpeg. progress вызывается при каждом обновлении прогресса.
+func Transcode(ctx context.Context, opts Options, progress func(Progress)) error {
 	if err := os.MkdirAll(filepath.Dir(opts.Output), 0o755); err != nil {
 		return err
 	}
@@ -42,20 +47,29 @@ func Transcode(ctx context.Context, opts Options, progress func(float64)) error
 	}
 
 	// Парсим прогресс из stdout (-progress pipe:1)
-	if opts.SourceInfo.DurationSec > 0 && progress != nil {
+	if progress != nil {
+		var cur Progress
 		scanner := bufio.NewScanner(stdout)
 		for scanner.Scan() {
 			line := scanner.Text()
-			if strings.HasPrefix(line, "out_time_us=") {
+			switch {
+			case strings.HasPrefix(line, "out_time_us="):
 				val := strings.TrimPrefix(line, "out_time_us=")
-				if us, err := strconv.ParseInt(val, 10, 64); err == nil && us > 0 {
+				if us, err := strconv.ParseInt(val, 10, 64); err == nil && us > 0 && opts.SourceInfo.DurationSec > 0 {
 					sec := float64(us) / 1e6
 					pct := sec / opts.SourceInfo.DurationSec
 					if pct > 1 {
 						pct = 1
 					}
-					progress(pct)
+					cur.Pct = pct
 				}
+			case strings.HasPrefix(line, "fps="):
+				val := strings.TrimPrefix(line, "fps=")
+				if fps, err := strconv.ParseFloat(strings.TrimSpace(val), 64); err == nil && fps > 0 {
+					cur.EncodeFPS = fps
+				}
+			case line == "progress=continue" || line == "progress=end":
+				progress(cur)
 			}
 		}
 	}