fix: file display, persistence, PDF support and save error handling

- Add file proxy route in maker blueprint (X-API-Key auth for browser requests)
- Persist file_path/annotations_json to DB via RecipeCreate/RecipeUpdate schemas
- Fix canvas sizing using grandparent container instead of Fabric.js wrapper div
- Defer canvas init with requestAnimationFrame for x-show timing
- Add PDF.js support in annotation-editor and annotation-viewer
- Fix annotations_json double-serialization (parse string to object before send)
- Handle FastAPI 422 validation error arrays in api_client and JS error display
- Update template URLs to use /maker/api/files/ proxy path

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

client/services/api_client.py

@@ -40,6 +40,11 @@ class APIClient:
         try:
             error_body = response.json()
             detail = error_body.get("detail", str(error_body))
+            # FastAPI 422 returns detail as a list of validation errors
+            if isinstance(detail, list):
+                detail = "; ".join(
+                    e.get("msg", str(e)) for e in detail if isinstance(e, dict)
+                ) or str(detail)
         except Exception:
             detail = response.text or f"HTTP {response.status_code}"