feat: Show empty workflow dialog when entering app builder with no nodes (#9379)

## Summary

Prompts users to load a template or return to graph when entering
builder mode on an empty workflow

## Screenshots (if applicable)

<img width="627" height="275" alt="image"
src="https://github.com/user-attachments/assets/c1a35dc3-4e8f-4abd-95b9-2f92524e8ebf"
/>

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-9379-feat-Show-empty-workflow-dialog-when-entering-app-builder-with-no-nodes-3196d73d36508123b643ec893cd86cac)
by [Unito](https://www.unito.io)

src/composables/useWorkflowTemplateSelectorDialog.test.ts

@@ -107,6 +107,32 @@ describe('useWorkflowTemplateSelectorDialog', () => {
       )
     })
 
+    it('invokes afterClose callback when dialog is closed', () => {
+      mockNewUserService.isNewUser.mockReturnValue(false)
+      const afterClose = vi.fn()
+
+      const dialog = useWorkflowTemplateSelectorDialog()
+      dialog.show('command', { afterClose })
+
+      const onClose =
+        mockDialogService.showLayoutDialog.mock.calls[0][0].props.onClose
+      onClose()
+
+      expect(mockDialogStore.closeDialog).toHaveBeenCalled()
+      expect(afterClose).toHaveBeenCalled()
+    })
+
+    it('does not fail when afterClose is not provided', () => {
+      mockNewUserService.isNewUser.mockReturnValue(false)
+
+      const dialog = useWorkflowTemplateSelectorDialog()
+      dialog.show('command')
+
+      const onClose =
+        mockDialogService.showLayoutDialog.mock.calls[0][0].props.onClose
+      expect(() => onClose()).not.toThrow()
+    })
+
     it('tracks telemetry with source', () => {
       mockNewUserService.isNewUser.mockReturnValue(false)
 

src/composables/useWorkflowTemplateSelectorDialog.ts

@@ -1,5 +1,6 @@
 import WorkflowTemplateSelectorDialog from '@/components/custom/widget/WorkflowTemplateSelectorDialog.vue'
 import { useTelemetry } from '@/platform/telemetry'
+import type { TemplateLibraryMetadata } from '@/platform/telemetry/types'
 import { useDialogService } from '@/services/dialogService'
 import { useNewUserService } from '@/services/useNewUserService'
 import { useDialogStore } from '@/stores/dialogStore'
@@ -17,8 +18,8 @@ export const useWorkflowTemplateSelectorDialog = () => {
   }
 
   function show(
-    source: 'sidebar' | 'menu' | 'command' = 'command',
-    options?: { initialCategory?: string }
+    source: TemplateLibraryMetadata['source'] = 'command',
+    options?: { initialCategory?: string; afterClose?: () => void }
   ) {
     useTelemetry()?.trackTemplateLibraryOpened({ source })
 
@@ -30,7 +31,10 @@ export const useWorkflowTemplateSelectorDialog = () => {
       key: DIALOG_KEY,
       component: WorkflowTemplateSelectorDialog,
       props: {
-        onClose: hide,
+        onClose: () => {
+          hide()
+          options?.afterClose?.()
+        },
         initialCategory
       }
     })