feat(FN-2843): auto-select quick chat default model

- Extend /models API responses to include optional defaultProvider/defaultModelId from global settings, including empty and error paths
- Update QuickChatFAB to prefer the configured default model on load and fall back to first-model selection only when no agents exist
- Move quick chat model tag/title-wrap inline styles into QuickChatFAB.css using dashboard design tokens
- Add QuickChatFAB tests covering default-model auto-selection with and without agents and legacy behavior when no default is configured

packages/dashboard/app/api/legacy.ts

@@ -1114,6 +1114,8 @@ export interface ModelsResponse {
   models: ModelInfo[];
   favoriteProviders: string[];
   favoriteModels: string[];
+  defaultProvider?: string;
+  defaultModelId?: string;
 }
 
 /** Fetch available AI models from the model registry along with favoriteProviders */