feat: 前端沙盒模型列表仅显示国内已备案模型

- models.ts 移除 GPT/Claude/Gemini/Llama，仅保留 domestic 模型
- DEFAULT_MODEL 改为 deepseek-v4-flash（原为 general）
- ModelSelector 改为从 /sandbox/models 获取可用模型列表

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

frontend/src/components/ui/model-selector.tsx

@@ -15,15 +15,15 @@ export function ModelSelector({ value, onChange, className = '' }: ModelSelector
   const [models, setModels] = useState<ModelOption[]>(FALLBACK_MODELS);
 
   useEffect(() => {
-    fetch(`${API_BASE}/public/models`)
+    fetch(`${API_BASE}/sandbox/models`)
       .then(r => r.json())
       .then(data => {
         if (data.items?.length) {
           setModels(data.items.map((m: any) => ({
             id: m.id,
-            label: m.name,
+            label: m.label,
             provider: m.provider,
-            desc: m.description || '',
+            desc: m.desc,
           })));
         }
       })