fix: canvas fits image exactly, chained annotation loading, arrow move tracking

- Canvas now sizes to match the scaled image dimensions (zero empty space)
  instead of using a fixed aspect ratio, fixing coordinate mismatch between
  editor and viewer
- Annotations load only after background image is ready via _pendingAnnotations
  pattern, preventing placement at wrong coordinates
- Arrow endpoints (arrowX1/Y1/X2/Y2) update on object:modified using
  transform delta, so moved arrows serialize at correct position
- Coordinate scaling on load: coordScale = currentImageScale / savedImageScale
  handles annotations saved at different screen widths

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

client/templates/maker/recipe_preview.html

@@ -436,7 +436,7 @@
 <script>
   pdfjsLib.GlobalWorkerOptions.workerSrc = 'https://cdnjs.cloudflare.com/ajax/libs/pdf.js/3.11.174/pdf.worker.min.js';
 </script>
-<script src="{{ url_for('static', filename='js/annotation-viewer.js') }}?v=4"></script>
+<script src="{{ url_for('static', filename='js/annotation-viewer.js') }}?v=6"></script>
 <script>
 /**
  * Recipe Preview - Minimal Alpine.js component