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>

components/page-shell.tsx

@@ -1,12 +1,17 @@
 import Header from "@/components/header";
 import Footer from "@/components/footer";
 import TaijiBackdrop from "@/components/layout/taiji-backdrop";
-import {
-  CONTENT_SHELL,
-  type ContentShellWidth,
-} from "@/lib/layout";
 import { cn } from "@/lib/utils";
 
+/** 必须在组件内写完整类名，Tailwind 才能生成对应 CSS */
+const SHELL_WIDTH = {
+  default: "w-full max-w-4xl px-4 sm:px-6 lg:px-8",
+  narrow: "w-full max-w-3xl px-4 sm:px-6 lg:px-8",
+  wide: "w-full max-w-4xl px-4 sm:px-6 lg:px-8",
+} as const;
+
+export type ContentShellWidth = keyof typeof SHELL_WIDTH;
+
 export default function PageShell({
   children,
   className = "",
@@ -23,7 +28,7 @@ export default function PageShell({
       <main
         className={cn(
           "mx-auto flex w-full flex-1 flex-col border-border/40 bg-background sm:border-x",
-          CONTENT_SHELL[width],
+          SHELL_WIDTH[width],
           className,
         )}
       >