feat: Replace BuilderExitButton with new BuilderFooterToolbar (#9378)

## Summary

Makes it easier and more obvious for users to navigate between steps

## Changes

- **What**: 
- add back/next navigation to builder footer alongside exit button
- extract shared step logic into useBuilderSteps composable

## Screenshots (if applicable)

<img width="428" height="102" alt="image"
src="https://github.com/user-attachments/assets/91b33e8f-53ae-4895-a2eb-fb1316b2b367"
/>

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-9378-feat-Replace-BuilderExitButton-with-new-BuilderFooterToolbar-3196d73d3650819392efc171cf277326)
by [Unito](https://www.unito.io)

src/views/GraphView.vue

@@ -16,7 +16,7 @@
     <template v-if="isBuilderMode">
       <BuilderToolbar />
       <BuilderMenu />
-      <BuilderExitButton />
+      <BuilderFooterToolbar />
     </template>
   </div>
 
@@ -91,7 +91,7 @@ import { useBottomPanelStore } from '@/stores/workspace/bottomPanelStore'
 import { useColorPaletteStore } from '@/stores/workspace/colorPaletteStore'
 import { useSidebarTabStore } from '@/stores/workspace/sidebarTabStore'
 import { electronAPI } from '@/utils/envUtil'
-import BuilderExitButton from '@/components/builder/BuilderExitButton.vue'
+import BuilderFooterToolbar from '@/components/builder/BuilderFooterToolbar.vue'
 import BuilderMenu from '@/components/builder/BuilderMenu.vue'
 import BuilderToolbar from '@/components/builder/BuilderToolbar.vue'
 import LinearView from '@/views/LinearView.vue'