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/static/js/annotation-viewer.js

@@ -172,7 +172,13 @@ function annotationViewer() {
 
       // Editor format (objects array from annotation-editor.js)
       if (this.annotations.objects) {
-        var annoScale = this.canvas.width / (this.annotations.width || this.canvas.width);
+        var previewScale = this.scale;
+        var imgScale = this.annotations.imageScale;
+        // v2+: use imageScale for correct canvas→image→preview mapping
+        // v1 fallback: annoScale = previewWidth / editorWidth
+        var annoScale = imgScale
+          ? (previewScale / imgScale)
+          : (this.canvas.width / (this.annotations.width || this.canvas.width));
         _drawEditorAnnotations(this.ctx, this.annotations, annoScale);
         return;
       }
@@ -397,7 +403,12 @@ function _drawPreview(canvas, ctx, source, srcW, srcH, annotations, maxHeight) {
   ctx.drawImage(source, 0, 0, canvas.width, canvas.height);
 
   if (annotations && annotations.objects) {
-    var annoScale = canvas.width / (annotations.width || srcW);
+    var imgScale = annotations.imageScale;
+    // v2+: use imageScale for correct canvas→image→preview mapping
+    // v1 fallback: annoScale = previewWidth / editorWidth
+    var annoScale = imgScale
+      ? (scale / imgScale)
+      : (canvas.width / (annotations.width || srcW));
     _drawEditorAnnotations(ctx, annotations, annoScale);
   } else if (annotations && annotations.markers) {
     _drawLegacyAnnotations(ctx, annotations, scale);