feat(mobile): improve model picker header, banner and filter toggles

[snehasishcodes]

Model picker UX rework in apps/mobile models.tsx:

• Move current-model display out of the header into a sticky sub-header banner with full name, provider, status, and screen-reader summary label.
• Show filtered model count in the header slot.
• Put filter/sort toggles in the search row; chips render only when toggled on, with expanded/selected accessibility states.

[snhsish]

Reviewed