feat(layout): 基础布局/导航/页脚/明暗不闪烁

- BaseLayout：字体三轨统持、viewport-fit=cover、theme-color 双套、skip-link
- Header：≤4 项导航纪律 + sticky 毛玻璃 + aria-current；搜索 icon 占位待 T8
- Footer：ICP 备案链接（法定保留）+ built-from 具名槽位待 T7
- ThemeScript：head 内联先于首帧，手选 > 系统偏好，防白闪

验证: pnpm astro build exit 0；dist/index.html 含 ICP 文案与跳到正文链

Spec: 2026-10-24_add-blog-skeleton
Co-authored-by: GLM-5.3

src/layouts/BaseLayout.astro

@@ -0,0 +1,44 @@
+---
+// 基础布局：字体/样式统一在此持有（页面不再各自引包）
+// 明暗不闪烁：ThemeScript 必须在 <head> 内联先于首帧执行
+import 'lxgw-wenkai-screen-webfont/lxgwwenkaiscreen.css';
+import '@fontsource/noto-sans-sc/400.css';
+import '@fontsource/noto-sans-sc/500.css';
+import '@fontsource/noto-sans-sc/700.css';
+import '@fontsource/jetbrains-mono/400.css';
+import '@fontsource/jetbrains-mono/700.css';
+import '../styles/global.css';
+import Header from '../components/Header.astro';
+import Footer from '../components/Footer.astro';
+import ThemeScript from '../components/ThemeScript.astro';
+
+interface Props {
+  title: string;
+  description?: string;
+  /** 正文轨：reading 书评纸墨 / tech 技术（body 级字体氛围） */
+  track?: 'reading' | 'tech';
+}
+const { title, description = 'wait4x 的博客：阅读 · 技术 · 摄影', track = 'tech' } = Astro.props;
+---
+
+<!doctype html>
+<html lang="zh" data-track={track}>
+  <head>
+    <meta charset="utf-8" />
+    <meta name="viewport" content="width=device-width, initial-scale=1.0, viewport-fit=cover" />
+    <meta name="generator" content={Astro.generator} />
+    <meta name="description" content={description} />
+    <meta name="theme-color" content="#ffffff" media="(prefers-color-scheme: light)" />
+    <meta name="theme-color" content="#191d23" media="(prefers-color-scheme: dark)" />
+    <title>{title}</title>
+    <ThemeScript />
+  </head>
+  <body>
+    <a class="skip-link" href="#main">跳到正文</a>
+    <Header />
+    <main id="main" class="container">
+      <slot />
+    </main>
+    <Footer />
+  </body>
+</html>