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/detect.go

@@ -34,13 +34,14 @@ func ProbeVideo(path string) (VideoInfo, error) {
 
 	var raw struct {
 		Streams []struct {
-			CodecType  string `json:"codec_type"`
-			CodecName  string `json:"codec_name"`
-			Width      int    `json:"width"`
-			Height     int    `json:"height"`
-			RFrameRate string `json:"r_frame_rate"`
-			BitRate    string `json:"bit_rate"`
-			Channels   int    `json:"channels"`
+			CodecType    string `json:"codec_type"`
+			CodecName    string `json:"codec_name"`
+			Width        int    `json:"width"`
+			Height       int    `json:"height"`
+			RFrameRate   string `json:"r_frame_rate"`
+			AvgFrameRate string `json:"avg_frame_rate"`
+			BitRate      string `json:"bit_rate"`
+			Channels     int    `json:"channels"`
 		} `json:"streams"`
 		Format struct {
 			Duration string `json:"duration"`
@@ -58,7 +59,12 @@ func ProbeVideo(path string) (VideoInfo, error) {
 			info.Codec = s.CodecName
 			info.Width = s.Width
 			info.Height = s.Height
-			info.FPS = parseFraction(s.RFrameRate)
+			// avg_frame_rate надёжнее для MJPEG и кодеков с нестандартным таймбейсом
+			fps := parseFraction(s.RFrameRate)
+			if avg := parseFraction(s.AvgFrameRate); avg > 0 && (fps <= 0 || fps > 120) {
+				fps = avg
+			}
+			info.FPS = fps
 			if br, err := strconv.ParseInt(s.BitRate, 10, 64); err == nil {
 				info.VideoBitrate = br
 			}