add folder, file nodes and major usability improvements

frontend/src/defaultWorkflow.js

@@ -0,0 +1,56 @@
+import { extractWorkflow } from './pngMetadata.js';
+
+const DEFAULT_WORKFLOW_CANDIDATES = [
+  { path: '/default-workflow.json', type: 'json' },
+  { path: '/default-workflow.png', type: 'png' },
+];
+
+async function loadCandidate(candidate, fetchImpl, extractWorkflowFn) {
+  let response;
+  try {
+    response = await fetchImpl(candidate.path, { cache: 'no-store' });
+  } catch {
+    return null;
+  }
+
+  const contentType = response.headers?.get?.('content-type') || '';
+  const isHtmlFallback = typeof contentType === 'string' && contentType.toLowerCase().includes('text/html');
+
+  if (!response.ok) {
+    if (response.status === 404 || response.status === 0) return null;
+    throw new Error(`Failed to load ${candidate.path} (${response.status})`);
+  }
+
+  if (candidate.type === 'json') {
+    if (isHtmlFallback) return null;
+    try {
+      return await response.json();
+    } catch {
+      throw new Error(`${candidate.path} is not valid JSON`);
+    }
+  }
+
+  if (isHtmlFallback) return null;
+  const workflow = await extractWorkflowFn(await response.blob());
+  if (!workflow) {
+    throw new Error(`${candidate.path} does not contain embedded workflow metadata`);
+  }
+  return workflow;
+}
+
+export async function loadDefaultWorkflowAsset({
+  fetchImpl = fetch,
+  extractWorkflowFn = extractWorkflow,
+} = {}) {
+  for (const candidate of DEFAULT_WORKFLOW_CANDIDATES) {
+    const workflow = await loadCandidate(candidate, fetchImpl, extractWorkflowFn);
+    if (workflow) {
+      return {
+        source: candidate.path,
+        format: candidate.type,
+        workflow,
+      };
+    }
+  }
+  return null;
+}