fix: dismiss welcome modal after loading a workflow via Open Project

loadWorkflow now sets showQuickstart: false so the modal doesn't
persist over a loaded project. Previously the isCanvasEmpty gate
handled this implicitly.

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

src/store/workflowStore.ts

@@ -1727,6 +1727,8 @@ const workflowStoreImpl: StateCreator<WorkflowStore> = (set, get) => ({
       useExternalImageStorage: savedConfig?.useExternalImageStorage ?? true,
       // Reset viewed comments when loading new workflow
       viewedCommentNodeIds: new Set<string>(),
+      // Dismiss welcome modal after loading a workflow
+      showQuickstart: false,
     });
 
     // Clear snapshot unless explicitly preserving (e.g., AI workflow generation)