Fix ultrawide layout: Tailwind was not generating max-width classes.

Move width utilities into scanned components as literal class names, add lib to Tailwind content and safelist. Verified max-w-4xl in build CSS and /learn HTML.

Co-authored-by: Cursor <cursoragent@cursor.com>

lib/layout.ts

@@ -1,22 +1,2 @@
-const pad = "px-4 sm:px-6 lg:px-8";
-
-/** 内容区宽度档位（带鱼屏居中，两侧留空） */
-export const CONTENT_SHELL = {
-  /** 常规页：约 896px */
-  default: `w-full max-w-4xl ${pad}`,
-  /** 首页、文章：约 768px */
-  narrow: `w-full max-w-3xl ${pad}`,
-  /** 双栏测算：约 960px */
-  wide: `w-full max-w-[960px] ${pad}`,
-} as const;
-
-export type ContentShellWidth = keyof typeof CONTENT_SHELL;
-
-/** 页眉 / 页脚 — 与最宽内容区对齐 */
-export const APP_CONTAINER = `mx-auto ${CONTENT_SHELL.wide}`;
-
-/** @deprecated 使用 PageShell width="narrow" */
-export const HOME_CONTAINER = `mx-auto ${CONTENT_SHELL.narrow}`;
-
-/** 双栏测算工作区 */
-export const MODE_CONTAINER = `mx-auto ${CONTENT_SHELL.wide}`;
+/** @deprecated 宽度类已移至各组件内完整书写，避免 Tailwind 漏扫 */
+export type ContentShellWidth = "default" | "narrow" | "wide";