Sessions that die before title generation (or predate model tracking)
rendered as 'Untitled · unknown · 0 msgs' — two placeholders stacked in
one row reads as breakage to Hermes Cloud users. Now:
- Session rows omit the model segment entirely when the store has no
model (no more 'unknown' + dangling separator).
- The Overview 'Recent Sessions' card falls back to the message preview
as the row label (italic, same treatment as the History list) instead
of a bare 'Untitled', and skips the duplicate preview paragraph when
the preview IS the label.
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