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/task_editor.html

@@ -1075,7 +1075,14 @@
 {% block extra_js %}
 <script src="https://cdnjs.cloudflare.com/ajax/libs/pdf.js/3.11.174/pdf.min.js"></script>
 <script>if(typeof pdfjsLib!=='undefined')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>
+// Force reload when page is restored from browser bfcache (back/forward navigation).
+// This ensures fresh data is displayed after editing annotations in task_drawing.
+window.addEventListener('pageshow', function (event) {
+  if (event.persisted) { window.location.reload(); }
+});
+</script>
 <script>
 function taskEditor() {
   return {
@@ -1126,6 +1133,29 @@ function taskEditor() {
       // Sort tasks by order_index on load
       this.tasks.sort((a, b) => (a.order_index || 0) - (b.order_index || 0));
 
+      // Check if returning from task_drawing with updated annotations
+      try {
+        var updatedRaw = sessionStorage.getItem('taskDrawingUpdated');
+        if (updatedRaw) {
+          sessionStorage.removeItem('taskDrawingUpdated');
+          var updated = JSON.parse(updatedRaw);
+          if (updated.taskId) {
+            var task = this.tasks.find(function(t) { return t.id === updated.taskId; });
+            if (task) {
+              // Merge updated annotations into local state
+              if (updated.annotations_json) {
+                task.annotations_json = (typeof updated.annotations_json === 'string')
+                  ? JSON.parse(updated.annotations_json) : updated.annotations_json;
+              }
+              // Auto-expand the edited task and render preview
+              this.expandedTask = task.id;
+              this.renderTaskPreview(task);
+              return; // skip default expansion logic
+            }
+          }
+        }
+      } catch (_e) { /* sessionStorage parse error - ignore */ }
+
       // Auto-expand the first task if there is only one
       if (this.tasks.length === 1) {
         this.expandedTask = this.tasks[0].id;