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>

messages/de.json

@@ -69,7 +69,8 @@
         "title": "Gelesene Bücher",
         "finishedAt": "Beendet am",
         "showMore": "{count} weitere anzeigen",
-        "showLess": "Weniger anzeigen"
+        "showLess": "Weniger anzeigen",
+        "empty": "In Hardcover fertig gelesene Bücher erscheinen hier automatisch."
       },
       "activity": {
         "idleStatus": "System im Leerlauf / Geist aktiv",

messages/en.json

@@ -70,7 +70,8 @@
         "title": "Read",
         "finishedAt": "Finished",
         "showMore": "{count} more",
-        "showLess": "Show less"
+        "showLess": "Show less",
+        "empty": "Books finished in Hardcover will appear here automatically."
       },
       "activity": {
         "idleStatus": "System Idle / Mind Active",