chore: remove VSCode extensions.json

Removed the .vscode/extensions.json file from the frontend directory.

This file contained workspace-level VSCode extension recommendations which are better managed:
- At user level through personal VSCode settings
- Through project README documentation
- Via devcontainer or editors preferences if needed

Cleanup reduces repository clutter and avoids imposing specific extension recommendations on contributors.

frontend/.vscode/extensions.json

@@ -1,5 +0,0 @@
-{
-  "recommendations": [
-    "svelte.svelte-vscode"
-  ]
-}