[3.x] Fix FOUC during SSR development#2911
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During SSR development, Vite normally injects CSS via JavaScript on the client side. The server-rendered HTML arrives before any JS executes, causing a flash of unstyled content (FOUC). This is particularly noticeable when using CSS frameworks or component-scoped styles.
This PR traverses Vite's SSR module graph starting from the entry point to collect all CSS dependencies, then generates
<link>tags that are prepended to theheadarray returned by the SSR render. The tags includedata-vite-dev-idattributes so Vite's client-side JS can deduplicate them and avoid injecting duplicate<style>tags.This only applies during development. In production, Vite's build process already handles CSS extraction correctly.