feat(admin): 平台商户成员管理改用抽屉，消除商户视角切换与全局状态污染

- 交互重构：在「平台商户」与「平台账号」点击成员管理时，不再跳转路由或改写 localStorage.merchant_id，改为滑出 MerchantMembersDrawer 抽屉
- 抽屉能力：支持查看指定商户成员列表、新增成员（新建账号或关联已有账号）、修改角色与启用状态、移除成员
- 顶栏清理：彻底移除顶栏右上角的「当前商户：xxx」标签与「退出商户视角」按钮，超管登录自动清理任何残留上下文
- 后端补齐：新增平台专属 /api/platform/merchants/:id/members 与 roles 接口，平台操作不再依赖 X-Merchant-ID
- 验证：tsc -b && vite build 成功；全量 go test ./... 包含路由与鉴权回归测试 100% 通过

backend/internal/handler/merchant_platform.go

@@ -180,3 +180,71 @@ func (h *MerchantHandler) AdminUpdateMerchantAccount(c *gin.Context) {
 	}
 	response.OK(c, account)
 }
+
+// ListPlatformMerchantMembers GET /api/platform/merchants/:id/members
+func (h *MerchantHandler) ListPlatformMerchantMembers(c *gin.Context) {
+	id, _ := strconv.ParseUint(c.Param("id"), 10, 64)
+	if id == 0 {
+		response.BadRequest(c, "商户 ID 无效")
+		return
+	}
+	members, err := h.merchantSvc.ListMembers(uint(id))
+	if err != nil {
+		response.ServerError(c, err.Error())
+		return
+	}
+	response.OK(c, members)
+}
+
+// UpdatePlatformMerchantMember PATCH /api/platform/merchants/:id/members/:mid
+func (h *MerchantHandler) UpdatePlatformMerchantMember(c *gin.Context) {
+	id, _ := strconv.ParseUint(c.Param("id"), 10, 64)
+	mid, _ := strconv.ParseUint(c.Param("mid"), 10, 64)
+	if id == 0 || mid == 0 {
+		response.BadRequest(c, "参数无效")
+		return
+	}
+	var req updateMemberReq
+	if err := c.ShouldBindJSON(&req); err != nil {
+		response.BadRequest(c, "参数错误：角色必填")
+		return
+	}
+	member, err := h.merchantSvc.UpdateMember(uint(id), uint(mid), service.UpdateMemberInput{
+		Role: req.Role, Status: req.Status, IsDefault: req.IsDefault,
+	}, middleware.GetUserID(c))
+	if err != nil {
+		response.BadRequest(c, err.Error())
+		return
+	}
+	response.OK(c, member)
+}
+
+// DeletePlatformMerchantMember DELETE /api/platform/merchants/:id/members/:mid
+func (h *MerchantHandler) DeletePlatformMerchantMember(c *gin.Context) {
+	id, _ := strconv.ParseUint(c.Param("id"), 10, 64)
+	mid, _ := strconv.ParseUint(c.Param("mid"), 10, 64)
+	if id == 0 || mid == 0 {
+		response.BadRequest(c, "参数无效")
+		return
+	}
+	if err := h.merchantSvc.RemoveMember(uint(id), uint(mid), middleware.GetUserID(c)); err != nil {
+		response.BadRequest(c, err.Error())
+		return
+	}
+	response.OK(c, nil)
+}
+
+// ListPlatformMerchantRoles GET /api/platform/merchants/:id/roles
+func (h *MerchantHandler) ListPlatformMerchantRoles(c *gin.Context) {
+	id, _ := strconv.ParseUint(c.Param("id"), 10, 64)
+	if id == 0 {
+		response.BadRequest(c, "商户 ID 无效")
+		return
+	}
+	roles, err := h.merchantSvc.ListRoles(uint(id))
+	if err != nil {
+		response.ServerError(c, err.Error())
+		return
+	}
+	response.OK(c, roles)
+}