fix: address review nits in load3d (#8779)

## Summary
- Refactor getModelUrl to use const instead of let
- add missing language key

improve for https://github.com/Comfy-Org/ComfyUI_frontend/pull/8765

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-8779-fix-address-review-nits-in-load3d-3036d73d36508183af11c5e9bc545650)
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src/locales/en/main.json

@@ -2387,6 +2387,7 @@
       "placeholderImage": "Select image...",
       "placeholderAudio": "Select audio...",
       "placeholderVideo": "Select video...",
+      "placeholderMesh": "Select mesh...",
       "placeholderModel": "Select model...",
       "placeholderUnknown": "Select media..."
     },