refactor(ai): 助手按 AI-AGENT 开发指南对齐(Ask 模式)

对照指南逐条评估后落地 4 项真实差距(MCP/Agent Loop/白名单等重型章节不适用,明确排除):
- 上下文预算(指南 07 TrimMessages):历史按 24k 字符预算从最旧裁剪,system 标注
  「更早的对话已省略」;抽 trimHistory 独立函数+单测(超预算丢最旧/小历史不裁)
- 配额超限诚实终态(指南 05.3/06 禁止伪装 completed):不再 400 报错,返回提示
  文案;前端渲染为 warning 提示条样式,对话上下文保留
- 回复脱敏(指南 08):assistant 回复同样过 RedactTextOnly,堵住模型复述样本
  带回敏感串的通道
- 退避重试(指南 05.2):429/5xx/网络错误指数退避 1s→2s 最多 2 次(计入总超时),
  鉴权/参数错不重试

frontend/src/components/AiAssistant.tsx

@@ -85,17 +85,22 @@ export const AiAssistant = memo(function AiAssistant() {
             {messages.length === 0 && (
               <p className="py-8 text-center text-xs text-muted-foreground">{t('assistant.emptyHint')}</p>
             )}
-            {messages.map((m, i) => (
-              <div key={i} className={m.role === 'user' ? 'flex justify-end' : 'flex justify-start'}>
-                <div className={
-                  m.role === 'user'
-                    ? 'max-w-[85%] rounded-lg bg-primary px-3 py-2 text-primary-foreground'
-                    : 'max-w-[85%] whitespace-pre-wrap rounded-lg bg-muted px-3 py-2'
-                }>
-                  {m.content}
+            {messages.map((m, i) => {
+              const isQuota = m.role === 'assistant' && m.content.includes('今日 AI 配额已用完')
+              return (
+                <div key={i} className={m.role === 'user' ? 'flex justify-end' : 'flex justify-start'}>
+                  <div className={
+                    isQuota
+                      ? 'max-w-[85%] whitespace-pre-wrap rounded-lg border border-warning/40 bg-warning/10 px-3 py-2 text-warning'
+                      : m.role === 'user'
+                        ? 'max-w-[85%] rounded-lg bg-primary px-3 py-2 text-primary-foreground'
+                        : 'max-w-[85%] whitespace-pre-wrap rounded-lg bg-muted px-3 py-2'
+                  }>
+                    {m.content}
+                  </div>
                 </div>
-              </div>
-            ))}
+              )
+            })}
             {sending && (
               <div className="flex justify-start">
                 <div className="flex items-center gap-2 rounded-lg bg-muted px-3 py-2 text-xs text-muted-foreground">