Before: the attachment chip just dimmed (opacity 0.6) while the file
downloaded — easy to miss on a large attachment.
Now: replace the paperclip SVG with a 12px whirlpool spinner for the
duration of the fetch, restoring the original icon when the download
finishes (or errors out). Same loading vocabulary as Test / Scan /
Probe / Send buttons elsewhere in the UI.
cd9ad1a7f2