refactor(web): 营销站重构为 Eleventy SSG，内容迁移至 Markdown

- 引入 Eleventy 静态站点生成器，页面统一使用 Nunjucks 模板
- base.njk 提供公共 nav / footer / CSS，消除三个页面间的重复代码
- 抽取 color.css（设计 token）+ style.css（公共组件 + utility 类）
- index.njk：首页迁移，用 utility class 替换大量页面私有 CSS
- download.njk：下载页重构，版本号/更新日志从 CHANGELOG.md 构建时解析，OS 自动识别
- docs.njk：文档页重构，12 章内容从 web/content/docs.md 构建时渲染，三列布局保留
- docs.css 单独提取，docs 专有样式不污染公共 CSS
- 内容面向非技术用户重写，去除技术术语和内部路径引用

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

web/_data/changelog.js

@@ -0,0 +1,44 @@
+const fs = require('fs');
+const path = require('path');
+
+module.exports = function() {
+  const raw = fs.readFileSync(path.join(__dirname, '../../CHANGELOG.md'), 'utf8');
+  const lines = raw.split('\n');
+  const versions = [];
+  let current = null;
+  let currentSection = null;
+
+  for (const line of lines) {
+    const versionMatch = line.match(/^## \[([^\]]+)\] - (\d{4}-\d{2}-\d{2})/);
+    if (versionMatch) {
+      if (current) versions.push(current);
+      if (versions.length >= 3) break;
+      current = { version: versionMatch[1], date: versionMatch[2], sections: [], intro: '' };
+      currentSection = null;
+      continue;
+    }
+
+    const sectionMatch = line.match(/^### (.+)/);
+    if (sectionMatch && current) {
+      currentSection = { type: sectionMatch[1], items: [] };
+      current.sections.push(currentSection);
+      continue;
+    }
+
+    const itemMatch = line.match(/^- (.+)/);
+    if (itemMatch && current) {
+      if (currentSection) {
+        currentSection.items.push(itemMatch[1]);
+      }
+      continue;
+    }
+
+    // Free-form text before first section (e.g. v1.0.0 intro paragraph)
+    if (line.trim() && current && !currentSection && !line.startsWith('#')) {
+      current.intro = current.intro ? current.intro + ' ' + line.trim() : line.trim();
+    }
+  }
+  if (current && versions.length < 3) versions.push(current);
+
+  return versions;
+};