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/footer.tsx

@@ -1,15 +1,14 @@
 import React from "react";
 import { VERSION } from "@/lib/constant";
-import { APP_CONTAINER } from "@/lib/layout";
 
 function Footer() {
   return (
-    <footer
-      className={`${APP_CONTAINER} relative z-10 flex items-center justify-center gap-1 py-4 text-xs text-muted-foreground/80`}
-    >
-      <span className="italic">心诚则灵</span>
-      <span className="text-muted-foreground/60">·</span>
-      <span>知命阁 v{VERSION}</span>
+    <footer className="relative z-10 flex items-center justify-center gap-1 border-t border-border/40 bg-card/70 py-4 text-xs text-muted-foreground/80">
+      <div className="mx-auto flex w-full max-w-4xl items-center justify-center gap-1 px-4 sm:px-6 lg:px-8">
+        <span className="italic">心诚则灵</span>
+        <span className="text-muted-foreground/60">·</span>
+        <span>知命阁 v{VERSION}</span>
+      </div>
     </footer>
   );
 }