fix(composite): async image decode, skip dead pager, prevent UI freeze

- Decode images in background goroutine, not in Draw()
- Show [loading image...] placeholder until decode completes
- Skip pv.term.Draw() when composite is active (pager got no input)
- Thread-safe access to decoded image cache via sync.Mutex
- Invalidate callback triggers redraw when images finish decoding

app/msgviewer.go

@@ -636,7 +636,8 @@ func (pv *PartViewer) attemptCopy() {
 			if len(images) > 0 {
 				pv.imageRefs = images
 				pv.composite = newCompositeContent(
-					string(cleanedBytes), images, 80)
+					string(cleanedBytes), images, 80,
+					pv.Invalidate)
 				pv.Invalidate()
 			} else {
 				// No images — forward to pager as normal
@@ -835,10 +836,8 @@ func (pv *PartViewer) Draw(ctx *ui.Context) {
 		ctx.Printf(0, 0, style, "%s", pv.err.Error())
 		return
 	}
-	if pv.term != nil {
-		pv.term.Draw(ctx)
-	}
 	// Composite mode: text + inline images from filter output
+	// Skip term.Draw() — pager got no input in composite mode
 	if pv.composite != nil {
 		pv.composite.width = ctx.Width()
 		style := pv.uiConfig.GetStyle(config.STYLE_DEFAULT)
@@ -846,6 +845,9 @@ func (pv *PartViewer) Draw(ctx *ui.Context) {
 		pv.composite.Draw(ctx, pv.scroll, style)
 		return
 	}
+	if pv.term != nil {
+		pv.term.Draw(ctx)
+	}
 	if pv.image != nil && (pv.resized(ctx) || pv.graphic == nil) {
 		// This path should only occur on resizes or the first pass
 		// after the image is downloaded and could be slow due to