fix: AI预测无反馈 + 增加推理建议展示

- useAIPredict 增加 markdown 代码块剥离逻辑
- Prompt 模板要求返回纯 JSON
- 可视化编辑器显示预测错误信息
- 展示 AI 推理建议（建议如何优化配方）

backend/src/services/ai/templates/index.ts

@@ -3,7 +3,7 @@ import type { ChatMessage } from '../providers/types.js'
 export function predictMetricsPrompt(ingredients: Array<{ name: string; percentage: number; category: string }>): ChatMessage[] {
   const ingList = ingredients.map(i => `- ${i.name} (${i.category}): ${i.percentage}%`).join('\n')
   return [
-    { role: 'system', content: '你是一名资深化妆品配方工程师。根据成分列表预测配方的肤感指数、稳定性评分和成本估算。返回 JSON 格式：{"sensoryIndex":{"spreadability":0-100,"absorption":0-100,"stickiness":0-100,"overall":0-100},"stabilityScore":0-100,"costEstimate":元/kg,"confidence":0-1,"reasoning":"简短理由"}' },
+    { role: 'system', content: '你是一名资深化妆品配方工程师。根据成分列表预测配方的肤感指数、稳定性评分和成本估算。直接返回纯 JSON，不要用 ``` 包裹：{"sensoryIndex":{"spreadability":0-100,"absorption":0-100,"stickiness":0-100,"overall":0-100},"stabilityScore":0-100,"costEstimate":元/kg,"confidence":0-1,"reasoning":"简短理由"}' },
     { role: 'user', content: `请分析以下配方的指标：\n${ingList}` },
   ]
 }