fix(msgviewer): use pager for all text, replace image markers with readable placeholders

The composite renderer bypassed the pager (less) for any email with images,
losing colors, scrolling, and search. Now always forward to the pager with
[image: alt] text placeholders. Image refs are still extracted and stored
for future inline rendering.

lib/parse/extract_images.go

@@ -39,3 +39,33 @@ func ExtractImages(r io.Reader) (io.Reader, []ImageRef) {
 	}
 	return buf, images
 }
+
+// ReplacePlaceholders converts \x00IMG:N\x00 placeholders into human-readable
+// [image: alt] text suitable for display in a pager.
+func ReplacePlaceholders(data []byte, images []ImageRef) []byte {
+	result := bytes.NewBuffer(nil)
+	for _, line := range bytes.Split(data, []byte("\n")) {
+		trimmed := bytes.TrimSpace(line)
+		if len(trimmed) > 6 && trimmed[0] == 0 &&
+			bytes.HasPrefix(trimmed, []byte("\x00IMG:")) &&
+			trimmed[len(trimmed)-1] == 0 {
+			// Extract index
+			numStr := string(trimmed[5 : len(trimmed)-1])
+			idx := 0
+			for _, c := range numStr {
+				if c >= '0' && c <= '9' {
+					idx = idx*10 + int(c-'0')
+				}
+			}
+			if idx < len(images) && images[idx].Alt != "" {
+				fmt.Fprintf(result, "[image: %s]\n", images[idx].Alt)
+			} else {
+				result.WriteString("[image]\n")
+			}
+		} else {
+			result.Write(line)
+			result.WriteByte('\n')
+		}
+	}
+	return result.Bytes()
+}