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/components/builder/EmptyWorkflowDialogContent.vue

@@ -0,0 +1,40 @@
+<template>
+  <BuilderDialog :show-close="false">
+    <template #title>
+      {{ $t('builderToolbar.emptyWorkflowTitle') }}
+    </template>
+
+    <div class="flex flex-col gap-2">
+      <p class="m-0 text-sm text-muted-foreground">
+        {{ $t('builderToolbar.emptyWorkflowExplanation') }}
+      </p>
+      <p class="m-0 text-sm text-muted-foreground">
+        {{ $t('builderToolbar.emptyWorkflowPrompt') }}
+      </p>
+    </div>
+
+    <template #footer>
+      <Button
+        variant="muted-textonly"
+        size="lg"
+        @click="$emit('backToWorkflow')"
+      >
+        {{ $t('builderToolbar.backToWorkflow') }}
+      </Button>
+      <Button variant="secondary" size="lg" @click="$emit('loadTemplate')">
+        {{ $t('builderToolbar.loadTemplate') }}
+      </Button>
+    </template>
+  </BuilderDialog>
+</template>
+
+<script setup lang="ts">
+import Button from '@/components/ui/button/Button.vue'
+
+import BuilderDialog from './BuilderDialog.vue'
+
+defineEmits<{
+  backToWorkflow: []
+  loadTemplate: []
+}>()
+</script>