feat(page): use topFullwidthBanner as og:image source

Pages now expose `socialImage` from the topFullwidthBanner field —
same 1200×630 cover transform with focal-aware crop that posts already
use. Layout meta-tag pipeline picks it up automatically; pages without
a banner still fall back to the global default social image.

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

src/routes/[...slug]/+page.server.ts

@@ -2,6 +2,7 @@ import type { PageServerLoad } from './$types';
 import { getPageBySlug } from '$lib/cms';
 import {
   extractCmsImageField,
+  ensureTransformedImage,
   resolveContentImages,
   resolveImageUrls,
 } from '$lib/rusty-image';
@@ -12,7 +13,8 @@ import {
   resolveCalendarBlocks,
   resolveDeadlineBannerBlocks,
 } from '$lib/blog-utils';
-import { PAGE_RESOLVE } from '$lib/constants';
+import { PAGE_RESOLVE, POST_SOCIAL_IMAGE_TRANSFORM } from '$lib/constants';
+import { env } from '$env/dynamic/public';
 import { error } from '@sveltejs/kit';
 import { sanitizeBlocks } from '$lib/sanitize-blocks.server';
 
@@ -69,7 +71,10 @@ export const load: PageServerLoad = async ({ params, locals, fetch, url, setHead
   let topBannerResolvedImages: string[] = [];
   let topBannerResponsive: ResponsiveImage | null = null;
   let topBannerFocalCss: string | null = null;
+  // Banner-Bild ist auch unsere Social-Image-Quelle (1200×630 cover, mit Focal).
+  let pageSocialImageAbs: string | null = null;
   const bannerRaw = (page as { topFullwidthBanner?: unknown }).topFullwidthBanner;
+  const siteBaseUrl = (env.PUBLIC_SITE_URL || '').replace(/\/$/, '');
   if (bannerRaw && typeof bannerRaw === 'object' && 'image' in bannerRaw) {
     const bannerImages = Array.isArray((bannerRaw as { image?: unknown }).image)
       ? ((bannerRaw as { image?: unknown[] }).image ?? [])
@@ -87,6 +92,14 @@ export const load: PageServerLoad = async ({ params, locals, fetch, url, setHead
           quality: 72,
           focal: primary.focal,
         });
+        const socialRel = ensureTransformedImage(primary.url, {
+          ...POST_SOCIAL_IMAGE_TRANSFORM,
+          focalX: primary.focal?.x,
+          focalY: primary.focal?.y,
+        });
+        pageSocialImageAbs = socialRel.startsWith('/')
+          ? `${siteBaseUrl}${socialRel}`
+          : socialRel;
       }
       topBannerResolvedImages = await resolveImageUrls(bannerImages as string[], {
         width: 2000,
@@ -105,6 +118,7 @@ export const load: PageServerLoad = async ({ params, locals, fetch, url, setHead
     previewDraft: !!previewDraft,
     seoTitle: page.seoTitle ?? page.headline ?? page.linkName ?? slug,
     seoDescription: page.seoDescription ?? page.subheadline ?? '',
+    socialImage: pageSocialImageAbs ?? undefined,
     robots: (page as { seoMetaRobots?: string }).seoMetaRobots ?? undefined,
     keywords: (page as { seoKeywords?: string }).seoKeywords ?? undefined,
     breadcrumbItems: [