/api/favicon returned 404 in three paths (negative cache hit, non-200
upstream, sub-10-byte body), and since the avatar loads it as <img src>,
the browser logged a red 404 for every sender domain without a public
favicon - dozens per inbox view. Now it returns HTTP 200 with a 1x1
transparent PNG and an X-Bulwark-Favicon: missing header. Avatar.tsx detects
the sentinel via naturalWidth <= 1 in onLoad and falls back to initials, so
behaviour is visually identical without the console noise.
