ui: loading bar below the model picker (#24931)

ui: loading bar below the model picker (#24931)

* ui: show model load progress on the selector trigger

Mirror the in-dropdown stage progress as a thin bar on the selector
trigger, so the active model's load percent stays visible when the menu
is closed. Same status gating and composite fraction as the dropdown
row, so both bars track the selected model in sync.

Suggested-by: Julien Chaumond <@julien-c>
* ui: show model load progress bar on the in-conversation model selector

* ui: tune model load indicator to a pulsing highlight (suggested by @ngxson)

Also wire the indicator onto the mobile sheet trigger, which was missing
it since mobile uses the sheet instead of the dropdown.

* ui: thin (@allozaur) pulsating (@ngxson) model load bar