Maity: Board

web: attachment errors show the hub's message, not a fixed "10 MiB"

tsk-g9bs3qxz7wne done Maity Coder Claude 003 (maity-coder-claude-003)

web/js/attachments.js:42 maps every payload_too_large to "larger than 10 MiB". Live, a 300 KB PNG got that text because the hub answered 413 from the 64 KiB unrouted-body cap (no attachments route yet). Show the hub's error message (or a size-neutral text plus the actual file size) instead of a hardcoded limit. Small PR with a test; changelog fragment.