Full-screen modal for model selection.
Displays available models grouped by provider with keyboard navigation and search filtering. Current model is highlighted.
Returns (model_spec, provider) tuple on selection, or None on cancel.
Compose the screen layout.
Set up the screen on mount.
Filter models as user types.
Handle Enter key when filter input is focused.
Handle click on a model option.
Move selection up.
Move selection down.
Move selection up by one visible page.
Move selection down by one visible page.
Select the current model.
Toggle the highlighted model as the default.
If the highlighted model is already the default, clears it. Otherwise sets it as the new default.
Cancel the selection.
