fix: add defer attribute to script tag

[hrobers3]

Hi there! First PR here — noticed a small improvement for frontend/index.html.

While type="module" scripts are deferred by specification, adding the explicit defer attribute makes the intent clear and documents the loading behavior for anyone reading the HTML. It is a one-line change with no functional impact (module scripts already defer), but it helps with code readability and documents the expected behavior at the markup level.

Before:

<script type="module" src="/src/main.tsx"></script>

After:

<script type="module" src="/src/main.tsx" defer></script>

Verified by re-reading the diff — the change is purely additive and does not affect script execution order.