Fix canvas initialization and sizing bugs

- Add image loading validation before sending canvas data to server
  Prevents 'Failed to get confirmation' error when images haven't
  finished loading after workflow reload. Waits 100ms and checks
  if all layer images are complete before rendering output.

- Improve layer loading error handling in CanvasState
  Better logging when layers fail to load from IndexedDB.
  Allows empty canvas as valid state instead of failing.

- Add ResizeObserver for canvas container
  Fixes bug where canvas only shows in top half of node.
  Watches container size changes and triggers re-render to ensure
  canvas dimensions are correctly calculated after DOM layout.

js/CanvasState.js

@@ -88,10 +88,10 @@ export class CanvasState {
             log.debug(`Output Area resized to ${this.canvas.width}x${this.canvas.height} and viewport set.`);
             const loadedLayers = await this._loadLayers(savedState.layers);
             this.canvas.layers = loadedLayers.filter((l) => l !== null);
-            log.info(`Loaded ${this.canvas.layers.length} layers.`);
-            if (this.canvas.layers.length === 0) {
-                log.warn("No valid layers loaded, state may be corrupted.");
-                return false;
+            log.info(`Loaded ${this.canvas.layers.length} layers from ${savedState.layers.length} saved layers.`);
+            if (this.canvas.layers.length === 0 && savedState.layers.length > 0) {
+                log.warn(`Failed to load any layers. Saved state had ${savedState.layers.length} layers but all failed to load. This may indicate corrupted IndexedDB data.`);
+                // Don't return false - allow empty canvas to be valid
             }
             this.canvas.updateSelectionAfterHistory();
             this.canvas.render();