fix: 平台消息身份传递 — AI-Core 收到正确昵称而非永远 fallback 到管理员

- forwardToAICore 新增 nickname 字段，格式 "昵称 (QQ号)" 明确标识发送者
- 解决非管理员用户 @昔涟 时 AI 仍认为是管理员的身份污染问题
- 同时包含：管理员群聊插入抑制、markdown 粗体剥离、SearXNG 容器、ethend 窗口隐藏

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/platform-bridge/internal/bridge/router.go

@@ -31,11 +31,13 @@ type PlatformRouter struct {
 
 // ChannelContext stores the active conversation state for a channel.
 type ChannelContext struct {
-	Platform    string
-	ChannelID   string
-	ChannelType string
-	LastUserMsg string
-	MessageCount int
+	Platform      string
+	ChannelID     string
+	ChannelType   string
+	LastUserMsg   string
+	LastSenderUID string
+	RecentSenders []string // last 5 sender UIDs (original platform UIDs)
+	MessageCount  int
 }
 
 func NewPlatformRouter(mapper *IdentityMapper, checker *permissions.Checker) *PlatformRouter {
@@ -207,6 +209,11 @@ func (r *PlatformRouter) updateContext(msg *UnifiedMessage) {
 		r.contexts[key] = ctx
 	}
 	ctx.LastUserMsg = msg.Content
+	ctx.LastSenderUID = msg.OriginalSenderUID
+	ctx.RecentSenders = append(ctx.RecentSenders, msg.OriginalSenderUID)
+	if len(ctx.RecentSenders) > 5 {
+		ctx.RecentSenders = ctx.RecentSenders[len(ctx.RecentSenders)-5:]
+	}
 	ctx.MessageCount++
 }