feat: replace text input with model picker for Fast Model in /settings (#3120)

The Fast Model setting in the settings dialog previously used a plain text
input, making it hard for users to discover available models. This replaces
it with the same model picker dialog used by `/model --fast`, adds a `▸`
visual indicator for sub-dialog settings, and supports right arrow to open
and left arrow to return.

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/cli/src/ui/contexts/UIActionsContext.tsx

@@ -61,6 +61,7 @@ export interface UIActions {
   exitEditorDialog: () => void;
   closeSettingsDialog: () => void;
   closeModelDialog: () => void;
+  openModelDialog: (options?: { fastModelMode?: boolean }) => void;
   openArenaDialog: (type: Exclude<ArenaDialogType, null>) => void;
   closeArenaDialog: () => void;
   handleArenaModelsSelected?: (models: string[]) => void;