fix: allow deleting system default sync channel with immediate heal

Remove hard ban that returned 400 with no UI feedback; delete then Ensure+rebinding, and show confirm/toast on SyncPage.

Co-authored-by: Cursor <cursoragent@cursor.com>

web/src/api.ts

@@ -869,6 +869,8 @@ export type SyncChannel = {
   pk_columns: Record<string, string>;
   /** 绑定智能体：模块/库归属对照 */
   agent_id?: number;
+  /** 公司默认同步通道（Z12） */
+  is_system_default?: boolean;
   /** 绑定已发布模块 slug */
   app_slug?: string;
   last_error?: string;
@@ -966,6 +968,16 @@ export async function deleteSyncChannel(session: Session, id: string) {
   });
   const data = await readJson(res);
   throwIfBad(res, data, "delete sync channel failed");
+  return data as {
+    ok?: boolean;
+    deleted_id?: string;
+    was_system_default?: boolean;
+    recreated_default?: boolean;
+    channel_id?: string;
+    bindings_fixed?: number;
+    agents_fixed?: number;
+    heal_error?: string;
+  };
 }
 
 export async function testSyncEndpoints(