fix: correct fallback status, clear stale params, and check tutorial fetch

- runWithFallback: use "loading" (valid NodeStatus) instead of "running"
- WorkflowCanvas: clear fallbackParameters when selecting a new fallback model
- workflowStore: add response.ok check before reading tutorial image blob

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

src/components/WorkflowCanvas.tsx

@@ -2499,6 +2499,7 @@ export function WorkflowCanvas() {
                 displayName: model.name,
                 capabilities: model.capabilities,
               },
+              fallbackParameters: {},
             });
             setFallbackDialogState(null);
           }}

src/store/execution/runWithFallback.ts

@@ -71,7 +71,7 @@ export async function runWithFallback(
   // Clear error state and stale output, show the fallback is now running.
   updateNodeData(nodeId, {
     ...clearOutput,
-    status: "running",
+    status: "loading",
     error: null,
     __usedFallback: true,
     __fallbackModelUsed: fallback.displayName,

src/store/workflowStore.ts

@@ -1673,6 +1673,7 @@ const workflowStoreImpl: StateCreator<WorkflowStore> = (set, get) => ({
     const mockImageUrl = "/tutorial/owl-aviator.png";
     try {
       const response = await fetch(mockImageUrl, { signal });
+      if (!response.ok) throw new Error(`Failed to fetch tutorial image: ${response.status}`);
       const blob = await response.blob();
       const reader = new FileReader();