feat: P0e 模型选择器升级 — 卡片式下拉

- ModelSelector 组件：标签/提供商/描述/选中状态
- 替换 sandbox page + workshop page 原生 select
- 模型数据增加 provider/desc 字段

frontend/src/app/prompts/workshop/page.tsx

@@ -5,6 +5,7 @@ import { useRouter } from 'next/navigation';
 import { useAuth } from '@/lib/auth-context';
 import { apiFetch, getToken } from '@/lib/auth';
 import { AVAILABLE_MODELS, DEFAULT_MODEL } from '@/lib/models';
+import { ModelSelector } from '@/components/ui/model-selector';
 
 const API_BASE = process.env.NEXT_PUBLIC_API_URL || 'http://localhost:3000/api/v1';
 
@@ -119,13 +120,7 @@ export default function PromptWorkshopPage() {
           <div className="bg-card rounded-2xl border border-border p-6">
             <div className="flex items-center justify-between mb-4">
               <h3 className="font-semibold text-foreground">提示词编辑</h3>
-              <select
-                value={model}
-                onChange={e => setModel(e.target.value)}
-                className="px-3 py-1.5 border border-border rounded-lg text-sm bg-background text-foreground focus:outline-none focus:ring-2 focus:ring-ring"
-              >
-                {AVAILABLE_MODELS.map(m => <option key={m.id} value={m.id}>{m.label}</option>)}
-              </select>
+              <ModelSelector value={model} onChange={setModel} className="w-48" />
             </div>
 
             <textarea

frontend/src/app/sandbox/page.tsx

@@ -5,6 +5,7 @@ import Link from 'next/link';
 import { useAuth } from '@/lib/auth-context';
 import { getToken, apiFetch } from '@/lib/auth';
 import { AVAILABLE_MODELS, DEFAULT_MODEL } from '@/lib/models';
+import { ModelSelector } from '@/components/ui/model-selector';
 import { useT } from '@/i18n';
 
 const API_BASE = process.env.NEXT_PUBLIC_API_URL || 'http://localhost:3000/api/v1';
@@ -254,10 +255,7 @@ export default function SandboxPage() {
           </div>
         </div>
         <div className="flex items-center gap-3">
-          <select value={model} onChange={e => setModel(e.target.value)}
-            className="px-3 py-1.5 border border-border rounded-lg text-sm bg-background focus:outline-none focus:border-brand-400">
-            {AVAILABLE_MODELS.map(m => <option key={m.id} value={m.id}>{m.label}</option>)}
-          </select>
+          <ModelSelector value={model} onChange={setModel} className="w-48" />
           {!isLoggedIn && (
             <Link href="/auth"
               className="px-4 py-1.5 text-sm font-medium text-brand-600 border border-brand-200 rounded-lg hover:bg-brand-50">