Improve command preview button and reorganize format options

Command Preview Button:
- Disabled when no video source is loaded
- Shows "Show Preview" when preview is hidden
- Shows "Hide Preview" when preview is visible
- Makes it clear when and why the button can be used

Format Options Reorganization:
- Grouped formats by codec family for better readability
- Order: H.264 → H.265 → AV1 → VP9 → ProRes → MPEG-2
- Added comments explaining each codec family
- Makes it easier to find and compare similar codecs

This improves discoverability and reduces user confusion about
when the command preview is available and which format to choose.

main.go

@@ -417,16 +417,22 @@ type formatOption struct {
 }
 
 var formatOptions = []formatOption{
+	// H.264 - Widely compatible, older standard
 	{"MP4 (H.264)", ".mp4", "libx264"},
-	{"MP4 (H.265)", ".mp4", "libx265"},
-	{"MP4 (AV1)", ".mp4", "libaom-av1"},
-	{"MKV (H.265)", ".mkv", "libx265"},
-	{"MKV (AV1)", ".mkv", "libaom-av1"},
-	{"WebM (VP9)", ".webm", "libvpx-vp9"},
-	{"WebM (AV1)", ".webm", "libaom-av1"},
 	{"MOV (H.264)", ".mov", "libx264"},
+	// H.265/HEVC - Better compression than H.264
+	{"MP4 (H.265)", ".mp4", "libx265"},
+	{"MKV (H.265)", ".mkv", "libx265"},
 	{"MOV (H.265)", ".mov", "libx265"},
+	// AV1 - Best compression, slower encode
+	{"MP4 (AV1)", ".mp4", "libaom-av1"},
+	{"MKV (AV1)", ".mkv", "libaom-av1"},
+	{"WebM (AV1)", ".webm", "libaom-av1"},
+	// VP9 - Google codec, good for web
+	{"WebM (VP9)", ".webm", "libvpx-vp9"},
+	// ProRes - Professional/editing codec
 	{"MOV (ProRes)", ".mov", "prores_ks"},
+	// MPEG-2 - DVD standard
 	{"DVD-NTSC (MPEG-2)", ".mpg", "mpeg2video"},
 	{"DVD-PAL (MPEG-2)", ".mpg", "mpeg2video"},
 }
@@ -5073,6 +5079,15 @@ func buildConvertView(state *appState, src *videoSource) fyne.CanvasObject {
 	})
 	cmdPreviewBtn.Importance = widget.LowImportance
 
+	// Update button text and state based on preview visibility and source
+	if src == nil {
+		cmdPreviewBtn.Disable()
+	} else if state.convertCommandPreviewShow {
+		cmdPreviewBtn.SetText("Hide Preview")
+	} else {
+		cmdPreviewBtn.SetText("Show Preview")
+	}
+
 	backBar := ui.TintedBar(convertColor, container.NewHBox(back, layout.NewSpacer(), navButtons, layout.NewSpacer(), cmdPreviewBtn, queueBtn))
 
 	var updateCover func(string)