Add audio channel remixing options to convert module

Added advanced audio channel remixing features for videos with imbalanced
left/right audio channels (e.g., music in left ear, vocals in right ear).

New audio channel options using FFmpeg pan filter:
- Left to Stereo: Copy left channel to both speakers
- Right to Stereo: Copy right channel to both speakers
- Mix to Stereo: Downmix both channels together evenly
- Swap L/R: Swap left and right channels

Changes:
- Updated audioChannelsSelect dropdown with 8 options (was 4)
- Implemented pan filter logic in all 4 FFmpeg command builders:
  - buildFFmpegCommandFromJob (main convert)
  - DVD encoding function
  - Convert command builder
  - Snippet generation
- Removed unused "os" import from internal/convert/ffmpeg.go
- Updated DONE.md with audio channel remixing feature

The pan filter syntax allows precise channel routing:
- pan=stereo|c0=c0|c1=c0 (left to both)
- pan=stereo|c0=c1|c1=c1 (right to both)
- pan=stereo|c0=0.5*c0+0.5*c1|c1=0.5*c0+0.5*c1 (mix)
- pan=stereo|c0=c1|c1=c0 (swap)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

DONE.md

@@ -103,6 +103,17 @@ This file tracks completed features, fixes, and milestones.
   - Better quality options now appear at top of list
   - Applied consistently to both simple and advanced modes
 
+- ✅ **Audio Channel Remixing** (2025-12-20 continuation)
+  - Added advanced audio channel options for videos with imbalanced L/R channels
+  - New options using FFmpeg pan filter:
+    - "Left to Stereo" - Copy left channel to both speakers (music only)
+    - "Right to Stereo" - Copy right channel to both speakers (vocals only)
+    - "Mix to Stereo" - Downmix both channels together evenly
+    - "Swap L/R" - Swap left and right channels
+  - Implemented in all 4 command builders (DVD, convert, snippet)
+  - Maintains existing options (Source, Mono, Stereo, 5.1)
+  - Solves problem of videos with music in one ear and vocals in the other
+
 ### Features (2025-12-18 Session)
 - ✅ **History Sidebar Enhancements**
   - Delete button ("×") on each history entry

internal/convert/ffmpeg.go

@@ -4,7 +4,6 @@ import (
 	"context"
 	"encoding/json"
 	"fmt"
-	"os"
 	"os/exec"
 	"path/filepath"
 	"strings"

main.go

@@ -3814,6 +3814,18 @@ func (s *appState) executeConvertJob(ctx context.Context, job *queue.Job, progre
 						args = append(args, "-ac", "2")
 					case "5.1":
 						args = append(args, "-ac", "6")
+					case "Left to Stereo":
+						// Copy left channel to both left and right
+						args = append(args, "-af", "pan=stereo|c0=c0|c1=c0")
+					case "Right to Stereo":
+						// Copy right channel to both left and right
+						args = append(args, "-af", "pan=stereo|c0=c1|c1=c1")
+					case "Mix to Stereo":
+						// Downmix both channels together, then duplicate to L+R
+						args = append(args, "-af", "pan=stereo|c0=0.5*c0+0.5*c1|c1=0.5*c0+0.5*c1")
+					case "Swap L/R":
+						// Swap left and right channels
+						args = append(args, "-af", "pan=stereo|c0=c1|c1=c0")
 					}
 				}
 
@@ -10495,6 +10507,18 @@ func (s *appState) startConvert(status *widget.Label, btn, cancelBtn *widget.But
 				args = append(args, "-ac", "2")
 			case "5.1":
 				args = append(args, "-ac", "6")
+			case "Left to Stereo":
+				// Copy left channel to both left and right
+				args = append(args, "-af", "pan=stereo|c0=c0|c1=c0")
+			case "Right to Stereo":
+				// Copy right channel to both left and right
+				args = append(args, "-af", "pan=stereo|c0=c1|c1=c1")
+			case "Mix to Stereo":
+				// Downmix both channels together, then duplicate to L+R
+				args = append(args, "-af", "pan=stereo|c0=0.5*c0+0.5*c1|c1=0.5*c0+0.5*c1")
+			case "Swap L/R":
+				// Swap left and right channels
+				args = append(args, "-af", "pan=stereo|c0=c1|c1=c0")
 			}
 		}
 
@@ -11103,6 +11127,18 @@ func (s *appState) generateSnippet() {
 				args = append(args, "-ac", "2")
 			case "5.1":
 				args = append(args, "-ac", "6")
+			case "Left to Stereo":
+				// Copy left channel to both left and right
+				args = append(args, "-af", "pan=stereo|c0=c0|c1=c0")
+			case "Right to Stereo":
+				// Copy right channel to both left and right
+				args = append(args, "-af", "pan=stereo|c0=c1|c1=c1")
+			case "Mix to Stereo":
+				// Downmix both channels together, then duplicate to L+R
+				args = append(args, "-af", "pan=stereo|c0=0.5*c0+0.5*c1|c1=0.5*c0+0.5*c1")
+			case "Swap L/R":
+				// Swap left and right channels
+				args = append(args, "-af", "pan=stereo|c0=c1|c1=c0")
 			}
 		}
 	}