Add arrow-key navigation to the model list

[acari-git]

Problem

Model selection is now exposed as a real Button, so Tab/Return/Space can reach individual selection controls. The model list still has no row-level Up/Down keyboard navigation for moving selection without tabbing through every control.

Current verification (2026-08-15)

• lightweightModelRow still uses row onTapGesture.
• modelSelectionButton is a Button and is accessible.
• No focused-selection state or Up/Down key handling was found for model rows.

Desired behavior

• Up/Down moves selection between visible model rows.
• Return/Space selects without starting or stopping a model.
• Sorting/filtering preserves deterministic navigation order.
• Add a UI/regression test that changes selectedModelID while leaving runtime state unchanged.