feat(web): 增加参数即时估价与显式免费编辑

估价请求统一使用 350ms 防抖、AbortController 和请求序号，区分免费、计算中、价格不可用与失败状态，并展示候选冻结上限。价格未就绪时阻止生产提交。\n\n计价规则编辑器新增显式免费开关和 9 位价格步进。\n\n验证：Web 88 项测试和生产构建通过。

apps/web/src/pages/PlaygroundPage.pricing.test.ts

@@ -0,0 +1,49 @@
+import { describe, expect, it } from 'vitest';
+import type { GatewayPricingEstimate } from '@easyai-ai-gateway/contracts';
+import {
+  MEDIA_ESTIMATE_DEBOUNCE_MS,
+  billingEstimateSignature,
+  mediaEstimateFromResponse,
+} from './PlaygroundPage';
+
+describe('playground pricing estimate', () => {
+  it('uses the specified 350ms debounce interval', () => {
+    expect(MEDIA_ESTIMATE_DEBOUNCE_MS).toBe(350);
+  });
+
+  it('only changes its signature for billing-relevant parameters', () => {
+    const first = billingEstimateSignature({
+      kind: 'images.generations', model: 'image-model', prompt: 'first',
+      count: 1, quality: 'high', resolution: '2K',
+    });
+    const promptOnly = billingEstimateSignature({
+      kind: 'images.generations', model: 'image-model', prompt: 'second',
+      count: 1, quality: 'high', resolution: '2K',
+    });
+    const changedCount = billingEstimateSignature({
+      kind: 'images.generations', model: 'image-model', prompt: 'second',
+      count: 2, quality: 'high', resolution: '2K',
+    });
+    expect(promptOnly).toBe(first);
+    expect(changedCount).not.toBe(first);
+  });
+
+  it('distinguishes explicit free and exposes the reservation ceiling', () => {
+    const response: GatewayPricingEstimate = {
+      candidateCount: 2,
+      currency: 'resource',
+      items: [{ amount: 0, currency: 'resource', isFree: true }],
+      pricingVersion: 'effective-pricing-v2',
+      requestFingerprint: 'fingerprint',
+      reservationAmount: 0,
+      resolver: 'effective-pricing-v2',
+      totalAmount: 0,
+    };
+    expect(mediaEstimateFromResponse(response)).toMatchObject({
+      amount: 0,
+      candidateCount: 2,
+      reservationAmount: 0,
+      status: 'free',
+    });
+  });
+});