feat(admin): 租户页加「共享计费」开关（每租户）

TenantRow 带 shared_billing；选定租户成员区上方加共享计费开关（PUT
/admin/tenants/:id/shared-billing）：开=成员消耗计本租户池、关=各计个人池。
live 验证：公司A 开关随真实状态回显为勾选。

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

sundynix-admin/src/api.ts

@@ -447,6 +447,7 @@ export interface TenantRow {
   plan: string;
   status: string;
   credit_balance_micro: number;
+  shared_billing: boolean;
   members: number;
 }
 export interface Member {
@@ -500,3 +501,11 @@ export async function removeMember(tenantId: string, uid: string): Promise<void>
     throw new Error(d.error ?? `remove member failed: ${res.status}`);
   }
 }
+
+export async function setSharedBilling(tenantId: string, on: boolean): Promise<void> {
+  const res = guard(await fetch(`${ADMIN}/tenants/${tenantId}/shared-billing`, { method: "PUT", headers: authHeaders(true), body: JSON.stringify({ shared_billing: on }) }));
+  if (!res.ok) {
+    const d = (await res.json().catch(() => ({}))) as { error?: string };
+    throw new Error(d.error ?? `set shared billing failed: ${res.status}`);
+  }
+}