feat: Hardcover→Directus book sync + fix empty states for projects/books

- Add POST /api/n8n/hardcover/sync-books — n8n calls this after detecting
  finished books in Hardcover. Authenticates via N8N_SECRET_TOKEN/N8N_API_KEY,
  deduplicates by hardcover_id, creates new book_reviews entries in Directus.

- Add getBookReviewByHardcoverId() + createBookReview() to lib/directus.ts.
  Check uses GraphQL filter; create uses Directus REST POST /items/book_reviews.

- ReadBooks: replace silent return null with a visible empty state so the
  section stays visible with a hint until the n8n sync populates it.

- Projects: add "No projects yet." placeholder instead of blank grid when
  both Directus and PostgreSQL return no data.

- Add home.about.readBooks.empty i18n key (EN + DE).

n8n workflow setup:
  Schedule → HTTP Hardcover GraphQL (books_read) → Code (transform) →
  POST /api/n8n/hardcover/sync-books with array of { hardcover_id, title,
  author, image, rating, finished_at }

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

app/components/Projects.tsx

@@ -74,6 +74,10 @@ const Projects = () => {
                 </div>
               </div>
             ))
+          ) : projects.length === 0 ? (
+            <div className="col-span-2 py-12 text-center text-stone-400 dark:text-stone-600 text-sm">
+              No projects yet.
+            </div>
           ) : (
             projects.map((project) => (
             <motion.div

app/components/ReadBooks.tsx

@@ -101,7 +101,12 @@ const ReadBooks = () => {
   }
 
   if (reviews.length === 0) {
-    return null; // Hier kannst du temporär "Keine Bücher gefunden" reinschreiben zum Testen
+    return (
+      <div className="flex items-center gap-3 py-4 text-sm text-stone-400 dark:text-stone-500">
+        <BookCheck size={16} className="shrink-0" />
+        <span>{t("empty")}</span>
+      </div>
+    );
   }
 
   const visibleReviews = expanded ? reviews : reviews.slice(0, INITIAL_SHOW);