feat(portfolio): real project-detail hero, no wasted space

The detail page rendered an empty <PageHero /> (blank gradient band) with the
real title buried in a card lower down (Phase 0 finding).

- Populate the detail PageHero with the project category (badge), title and
  summary, so the top of the page is a proper header.
- Slim the in-content ProjectHeader to back-link + meta chips + preview button,
  removing the now-duplicated title and summary. Applies across all three
  detail templates (grid/story/case-study).

Verified: eslint clean, no new type errors.

app/[locale]/(site)/portfolio/[slug]/page.tsx

@@ -58,10 +58,18 @@ export default async function PortfolioItemPage({
   }
 
   const t = await getTranslations({ locale: localeKey, namespace: "portfolioDetail" });
+  const title = getLocalizedValue(item.title, localeKey);
+  const category = getLocalizedValue(item.category.name, localeKey);
+  const summary = getLocalizedValue(item.summary, localeKey);
 
   return (
     <>
-      <PageHero />
+      <PageHero
+        locale={localeKey}
+        badge={category}
+        lines={[{ text: title, tone: "soft", align: "center" }]}
+        description={summary}
+      />
 
       <Container size="wide" className="pb-12 lg:pb-16">
         <PortfolioProjectDetail