Narrow desktop layout margins and fix taiji icon in light mode.

Cap content width at 1152px with side margins on ultrawide screens, add stroke to taiji logo for light backgrounds, and shrink backdrop watermark.

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

lib/layout.ts

@@ -1,3 +1,10 @@
-/** 全站内容区宽度：手机满宽，带鱼屏最大约 1920px 并留边距 */
-export const APP_CONTAINER =
-  "mx-auto w-full max-w-[min(100%,1920px)] px-4 sm:px-6 lg:px-8 xl:px-12 2xl:px-16";
+const CENTER = "mx-auto w-full px-4 sm:px-6 lg:px-8";
+
+/** 页眉 / 页脚 / 学习 / 测算页标题 — 居中留白，最大约 1152px */
+export const APP_CONTAINER = `${CENTER} max-w-6xl`;
+
+/** 首页单列 — 更窄一些 */
+export const HOME_CONTAINER = `${CENTER} max-w-4xl`;
+
+/** 双栏测算工作区 — 与全站同宽，避免带鱼屏拉满 */
+export const MODE_CONTAINER = `${CENTER} max-w-6xl xl:px-10`;