merge: integrate upstream v0.1.95 with our customizations

Merge main (custom features) into release/custom-0.1.95 (upstream v0.1.95).
New upstream features: group subscription binding, multi-dimension quota (daily/weekly/total),
allow_messages_dispatch, default_mapped_model, recover state API.
Our customizations: simulate_claude_max_enabled, usage status detection, 403 validation handling.

backend/internal/handler/admin/account_handler.go

@@ -1337,6 +1337,12 @@ func (h *AccountHandler) BulkUpdate(c *gin.Context) {
 			c.JSON(409, gin.H{
 				"error":   "mixed_channel_warning",
 				"message": mixedErr.Error(),
+				"details": gin.H{
+					"group_id":         mixedErr.GroupID,
+					"group_name":       mixedErr.GroupName,
+					"current_platform": mixedErr.CurrentPlatform,
+					"other_platform":   mixedErr.OtherPlatform,
+				},
 			})
 			return
 		}