* feat(ui): restructure chat transcript for multi-user sessions
Viewer-relative alignment: attributed messages from other participants
(senderId != viewer) render left-aligned as peers with their avatar,
name, and identity tint; only the viewer's own messages stay right-aligned.
System-role transcript entries (e.g. local command output) now render as
centered notice rows instead of a pseudo-participant with a question-mark
avatar. In threads with 2+ attributed senders, assistant replies carry a
'Replying to <name>' attribution chip derived from the preceding attributed
user turn; unattributed turns clear the attribution rather than mislabeling.
Also drops redundant role lowercasing on already-normalized roles.
* docs(web): describe multi-user chat transcript layout
* docs(web): refresh chat transcript docs map
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