React Router v7 Suspense streaming support

[azami]

Problem

React Router v7 Suspense/Await pattern does not display fallback components during data loading in development mode.

Root Cause

Missing HTTP streaming headers prevent proper Suspense functionality. React Router v7 requires transfer-encoding: chunked header for HTML responses to enable streaming behavior.

Current Workaround

Setting transfer-encoding: chunked for all HTML responses works but is overly broad and may cause unintended side effects.