fix: 操作日志详细记录权限变更

- 修改用户权限时记录旧角色→新角色（中文）和用户名
- 日志显示"查看者 → 高级编辑"格式
- 商业认证日志显示商户名

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/views/AuditLog.vue

@@ -153,9 +153,10 @@ function parsedDetail(log) {
   try {
     const d = JSON.parse(log.detail)
     const parts = []
+    if (d.from_role && d.to_role) parts.push(`${d.from_role} → ${d.to_role}`)
     if (d.from_user) parts.push(`来自: ${d.from_user}`)
     if (d.reason) parts.push(`原因: ${d.reason}`)
-    if (d.role) parts.push(`角色: ${d.role}`)
+    if (d.business_name) parts.push(`商户: ${d.business_name}`)
     if (d.display_name) parts.push(`显示名: ${d.display_name}`)
     if (d.original_log_id) parts.push(`恢复自 #${d.original_log_id}`)
     if (parts.length) return parts.join(' · ')