Implement DVD format FFmpeg codec selection and settings

Critical fix: When a DVD format (NTSC or PAL) is selected, now properly
override the video and audio codec to use DVD-compliant standards:

Video:
- Forces MPEG-2 codec (mpeg2video)
- NTSC: 6000k bitrate, 9000k max, gop=15
- PAL: 8000k bitrate, 9500k max, gop=12

Audio:
- Forces AC-3 codec for DVD container compatibility
- 192 kbps bitrate
- 48 kHz sample rate (DVD standard)
- Stereo channels (2)

This ensures that selecting a DVD format produces DVDStyler-compatible
MPEG files without codec errors. Previously, the code was using the
default H.264 + AAC, which caused 'unsupported audio codec' errors
when trying to write to MPEG container.

Fixes the issue where DVD conversions were failing with:
  'Unsupported audio codec. Must be one of mp1, mp2, mp3, 16-bit pcm_dvd,
   pcm_s16be, ac3 or dts.'

main.go

@@ -890,19 +890,38 @@ func (s *appState) executeConvertJob(ctx context.Context, job *queue.Job, progre
 		args = append(args, "-vf", strings.Join(vf, ","))
 	}
 
+	// Check if this is a DVD format (special handling required)
+	selectedFormat, _ := cfg["selectedFormat"].(formatOption)
+	isDVD := selectedFormat.Ext == ".mpg"
+
 	// Video codec
 	videoCodec, _ := cfg["videoCodec"].(string)
-	if videoCodec == "Copy" {
+	if videoCodec == "Copy" && !isDVD {
 		args = append(args, "-c:v", "copy")
 	} else {
 		// Determine the actual codec to use
-		actualCodec := determineVideoCodec(convertConfig{
+		var actualCodec string
+		if isDVD {
+			// DVD requires MPEG-2 video
+			actualCodec = "mpeg2video"
+		} else {
+			actualCodec = determineVideoCodec(convertConfig{
 				VideoCodec:    videoCodec,
 				HardwareAccel: hardwareAccel,
 			})
+		}
 		args = append(args, "-c:v", actualCodec)
 
-		// Bitrate mode and quality
+		// DVD-specific video settings
+		if isDVD {
+			// NTSC vs PAL settings
+			if strings.Contains(selectedFormat.Label, "NTSC") {
+				args = append(args, "-b:v", "6000k", "-maxrate", "9000k", "-bufsize", "1835k", "-g", "15")
+			} else if strings.Contains(selectedFormat.Label, "PAL") {
+				args = append(args, "-b:v", "8000k", "-maxrate", "9500k", "-bufsize", "2228k", "-g", "12")
+			}
+		} else {
+			// Standard bitrate mode and quality for non-DVD
 			bitrateMode, _ := cfg["bitrateMode"].(string)
 			if bitrateMode == "CRF" || bitrateMode == "" {
 				crfStr, _ := cfg["crf"].(string)
@@ -933,15 +952,28 @@ func (s *appState) executeConvertJob(ctx context.Context, job *queue.Job, progre
 				args = append(args, "-pix_fmt", pixelFormat)
 			}
 		}
+	}
 
 	// Audio codec and settings
 	audioCodec, _ := cfg["audioCodec"].(string)
-	if audioCodec == "Copy" {
+	if audioCodec == "Copy" && !isDVD {
 		args = append(args, "-c:a", "copy")
 	} else {
-		actualAudioCodec := determineAudioCodec(convertConfig{AudioCodec: audioCodec})
+		var actualAudioCodec string
+		if isDVD {
+			// DVD requires AC-3 audio
+			actualAudioCodec = "ac3"
+		} else {
+			actualAudioCodec = determineAudioCodec(convertConfig{AudioCodec: audioCodec})
+		}
 		args = append(args, "-c:a", actualAudioCodec)
 
+		// DVD-specific audio settings
+		if isDVD {
+			// DVD standard: AC-3 stereo at 48 kHz, 192 kbps
+			args = append(args, "-b:a", "192k", "-ar", "48000", "-ac", "2")
+		} else {
+			// Standard audio settings for non-DVD
 			if audioBitrate, _ := cfg["audioBitrate"].(string); audioBitrate != "" && actualAudioCodec != "flac" {
 				args = append(args, "-b:a", audioBitrate)
 			}
@@ -957,6 +989,7 @@ func (s *appState) executeConvertJob(ctx context.Context, job *queue.Job, progre
 				}
 			}
 		}
+	}
 
 	// Map cover art
 	if hasCoverArt {
@@ -965,8 +998,7 @@ func (s *appState) executeConvertJob(ctx context.Context, job *queue.Job, progre
 		args = append(args, "-disposition:v:1", "attached_pic")
 	}
 
-	// Format-specific settings
+	// Format-specific settings (already parsed above for DVD check)
-	var selectedFormat formatOption
 	switch v := cfg["selectedFormat"].(type) {
 	case formatOption:
 		selectedFormat = v