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.

components/site/portfolio-project-detail.tsx

@@ -235,14 +235,7 @@ function ProjectHeader({
             <Link href={getLocalizedPath(locale, "/portfolio", defaultLocale)}>{t("back")}</Link>
           </Button>
 
-          <h1 className="mt-5 text-3xl font-semibold text-foreground sm:text-4xl lg:text-5xl">
-            {getLocalizedValue(item.title, locale)}
-          </h1>
-          <p className="mt-4 max-w-3xl text-base leading-8 text-muted-foreground sm:text-lg">
-            {getLocalizedValue(item.summary, locale)}
-          </p>
-
-          <div className="mt-8">
+          <div className="mt-6">
             <ProjectMeta item={item} locale={locale} />
           </div>