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>

lib/directus.ts

@@ -993,3 +993,83 @@ export async function getSnippets(limit = 10, featured?: boolean): Promise<Snipp
     return null;
   }
 }
+
+// ─── Hardcover → Directus sync helpers ───────────────────────────────────────
+
+export interface BookReviewCreate {
+  hardcover_id: string;
+  book_title: string;
+  book_author: string;
+  book_image?: string;
+  rating?: number;
+  finished_at?: string;
+  status: 'published' | 'draft';
+}
+
+/**
+ * Check if a book review already exists in Directus by Hardcover ID.
+ * Used for deduplication during sync.
+ */
+export async function getBookReviewByHardcoverId(
+  hardcoverId: string
+): Promise<{ id: string } | null> {
+  const query = `
+    query {
+      book_reviews(
+        filter: { hardcover_id: { _eq: "${hardcoverId}" } }
+        limit: 1
+      ) {
+        id
+      }
+    }
+  `;
+
+  try {
+    const result = await directusRequest<{ book_reviews: Array<{ id: string }> }>(
+      '',
+      { body: { query } }
+    );
+    const item = result?.book_reviews?.[0];
+    return item ?? null;
+  } catch {
+    return null;
+  }
+}
+
+/**
+ * Create a new book review in the Directus book_reviews collection via REST API.
+ * Returns the created item id, or null on failure.
+ */
+export async function createBookReview(
+  data: BookReviewCreate
+): Promise<{ id: string } | null> {
+  if (!DIRECTUS_TOKEN) return null;
+
+  try {
+    const response = await fetch(`${DIRECTUS_URL}/items/book_reviews`, {
+      method: 'POST',
+      headers: {
+        'Content-Type': 'application/json',
+        Authorization: `Bearer ${DIRECTUS_TOKEN}`,
+      },
+      body: JSON.stringify(data),
+      signal: AbortSignal.timeout(5000),
+    });
+
+    if (!response.ok) {
+      const text = await response.text();
+      if (process.env.NODE_ENV === 'development') {
+        console.error(`Directus create book_review failed ${response.status}:`, text.slice(0, 200));
+      }
+      return null;
+    }
+
+    const result = await response.json();
+    return result?.data?.id ? { id: result.data.id } : null;
+  } catch (error) {
+    if (process.env.NODE_ENV === 'development') {
+      console.error('createBookReview error:', error);
+    }
+    return null;
+  }
+}