Sometimes the user lands in chat mode without realizing — surface the
mode the message went out on as a small uppercase pill right after the
timestamp in the role header.
- roleTimestamp(when, mode) gains an optional mode arg. Agent renders
in accent; Chat renders in muted/neutral. Other values render
nothing (back-compat for older history without the field).
- The three roleTimestamp call sites pass metadata?.mode through.
- chat.js writes mode into the user-message metadata at send time and
into the assistant metadata when the active-stream render lands,
reading toggleState.mode so research/agent overrides upstream still
flow through correctly.
Historical messages from before this change just don't show the pill —
graceful fallback, no migration needed.
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