- Add .model-opt-provider chip (right-aligned, muted) on every model row
that belongs to a provider group, making same-name models across
providers visually distinguishable at a glance.
- Add per-group model count to group headings: 'OpenRouter (47)'.
- Add subtle border-top divider between provider groups for visual
separation during scroll.
Scope: Shape A from #1425 — smallest change, ~15 LOC, no API churn.
Note: Settings model picker is a native <select> and already has optgroup
labels; this targets the custom dropdown used in the composer.
Closes#1425
6173d6d0ea
