feat: 修复宽屏下正文+TOC整体偏右、顶栏与TOC错位的问题

- 新增 TOC_BREAKPOINT 常量替代硬编码 1536，通过 define:vars 注入客户端脚本
- MainGridLayout 平移正文/TOC 容器让整体视觉居中，并用 margin 计算修复 Navbar 与 TOC 右边缘的 1rem 错位
- data-has-toc 同时判断 siteConfig.toc.enable 与 headings.length，避免 toc 关闭时仍触发平移
- 新增 adaptive-width-for-fuwari.md 教程文档

src/layouts/Layout.astro

@@ -15,6 +15,7 @@ import {
 	DEFAULT_THEME,
 	LIGHT_MODE,
 	PAGE_WIDTH,
+	TOC_BREAKPOINT,
 } from "../constants/constants";
 import { defaultFavicons } from "../constants/icon";
 import type { Favicon } from "../types/config";
@@ -572,10 +573,10 @@ if (window.swup) {
 }
 </script>
 
-<script>
+<script is:inline define:vars={{ TOC_BREAKPOINT }}>
 function adjustTocLayout() {
 	const hasToc = !!document.querySelector('[data-has-toc]');
-	const isWide = window.innerWidth >= 1536;
+	const isWide = window.innerWidth >= TOC_BREAKPOINT;
 	if (hasToc && isWide) {
 		document.body.classList.add('toc-offset');
 	} else {