Delete the two byte-identical local formatBytes copies from the chat-attachment
modules and repoint all call sites at the canonical lib/utils/format-bytes.ts,
which its own doc comment asks new code to use.
The copies had no upper clamp (>= 1 TB rendered "1 undefined") and no NaN or
negative guard (NaN rendered "NaN undefined"). Size labels now carry one
decimal ("10.0 MB" instead of "10 MB") because the canonical helper does not
strip a trailing zero; unit and e2e expectations updated to match.
Henry