Add support for per-model configuration

A provider's models may now be provided in the config as a simple
string, or as a mapping with at least a `name` key.

This is a valid configuration:

```yaml
models:
- name: model-a
  reasoning: true
- model-b
```

This opens the door to providing model-specific configuration, e.g. an
overridden max tokens count, or whether the model is a 'reasoning'
model.

pkg/tui/views/settings/settings.go

@@ -103,10 +103,10 @@ func (m *Model) getModelOptions() []list.OptionGroup {
 		group := list.OptionGroup{
 			Name: providerLabel,
 		}
-		for _, model := range p.Models {
+		for _, model := range p.Models() {
 			group.Options = append(group.Options, list.Option{
-				Label: model,
-				Value: modelOpt{provider, model},
+				Label: model.Name,
+				Value: modelOpt{provider, model.Name},
 			})
 		}
 		modelOpts = append(modelOpts, group)