Enable RSC Framework Mode

[brookslybrand]

This turns on React Router 8.4's unstable RSC Framework Mode with as few changes as possible. No page components were changed. They all still render the way they did before. The server component work is in #258.

What changed:

• Added @vitejs/plugin-rsc and new entry.rsc.tsx / entry.ssr.tsx files, replacing entry.server.tsx. server.ts now hands requests to the RSC build's fetch handler.
• Removed <Scripts /> from the root layout and dropped splitRouteModules from the config, since RSC mode doesn't support either.
• Moved getColorScheme and the ColorScheme type into color-scheme/utils.ts so the server action doesn't import from a "use client" file.
• The SSR entry passes non-Flight responses straight through, so .md URLs still return raw Markdown.
• Swapped <Await> for use() on the homepage stats. <Await> doesn't handle RSC promises yet, so the stats were never server-rendered and the browser logged hydration errors.

Everything else should work the same as before: middleware, caching, waiting for full content for bots, the stream timeout, error logging, SRI, and .server import protection. I smoke tested a production build: homepage, docs, the brand page, .md URLs, HEAD requests, and the theme toggle.

RSC Framework Mode is still unstable. #237 was useful background, but where it differed from the 8.4 RSC docs I followed the docs.