Add responsive content width for ultrawide while keeping 1080p layout.

Scale from 896px on 1080p up to 1440px on 3440px screens via viewport breakpoints in site-width.

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

components/layout/site-width.ts

@@ -0,0 +1,18 @@
+/**
+ * 响应式内容宽度（类名须完整书写，供 Tailwind 扫描）
+ * - ≤1920px（1080p）：约 896px
+ * - 1921px+（超宽入门）：约 1152px
+ * - 2560px+（带鱼屏）：约 1280px
+ * - 3440px+（超宽带鱼）：约 1440px
+ */
+export const SITE_PAD = "px-4 sm:px-6 lg:px-8";
+
+export const SITE_WIDTH =
+  `w-full max-w-4xl ${SITE_PAD} min-[1921px]:max-w-6xl min-[2560px]:max-w-7xl min-[3440px]:max-w-[1440px]`;
+
+export const SITE_WIDTH_NARROW =
+  `w-full max-w-3xl ${SITE_PAD} min-[1921px]:max-w-4xl min-[2560px]:max-w-5xl min-[3440px]:max-w-6xl`;
+
+/** 页眉 / 页脚内层，与主内容同宽 */
+export const SITE_WIDTH_INNER =
+  `mx-auto w-full max-w-4xl ${SITE_PAD} min-[1921px]:max-w-6xl min-[2560px]:max-w-7xl min-[3440px]:max-w-[1440px]`;