Fix Quartz layout export name to match v4.4.0 expected import

Rename sharedLayout to sharedPageComponents to fix build errors in
Quartz emitters (contentPage, tagPage, folderPage, 404).

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

quartz/quartz.layout.ts

@@ -2,7 +2,7 @@ import { PageLayout, SharedLayout } from "./quartz/cfg"
 import * as Component from "./quartz/components"
 
 // components shared across all pages
-export const sharedLayout: SharedLayout = {
+export const sharedPageComponents: SharedLayout = {
   head: Component.Head(),
   header: [],
   afterBody: [],