docs: add comprehensive color contrast and heading hierarchy optimization report with deployment guide

docs/color-heading-optimization-report.md

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+# 颜色对比度和标题层级优化报告
+## 项目概述
+本报告记录了Novalon网站的颜色对比度和标题层级结构优化工作，旨在提升网站的可访问性（WCAG 2.1 AA标准）和SEO性能。

tests/styles/color-contrast.spec.ts

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+import { test, expect } from '@playwright/test';
+import { calculateContrastRatio, meetsWCAGStandard } from '@/lib/color-contrast';
+
+test('primary text on primary background should meet WCAG AA', () => {
+  const result = meetsWCAGStandard('#1C1C1C', '#FAFAFA', 'AA', 'normal');
+  expect(result.passes).toBe(true);
+});
+
+test('tertiary text on primary background should meet WCAG AA', () => {
+  const result = meetsWCAGStandard('#5C5C5C', '#FAFAFA', 'AA', 'normal');
+  expect(result.passes).toBe(true);
+});
+
+test('muted text on primary background should meet WCAG AA', () => {
+  const result = meetsWCAGStandard('#8C8C8C', '#FAFAFA', 'AA', 'normal');
+  expect(result.passes).toBe(true);
+});