fix(website): migrate blog from next-mdx-remote to @next/mdx

next-mdx-remote evaluates MDX at runtime with a different React version
than Next.js 16's internal canary, causing build failures. Switch to
@next/mdx which compiles MDX through Next.js's own bundler pipeline.

- Replace next-mdx-remote with @next/mdx, @mdx-js/mdx, @mdx-js/loader
- Use dynamic imports for blog MDX files instead of MDXRemote
- Add remark-frontmatter so compiler skips YAML frontmatter blocks
- Gate cache/CSP headers to production only (immutable dev chunks
  were poisoning browser cache)

Made-with: Cursor

apps/website/mdx-components.tsx

@@ -0,0 +1,6 @@
+import type { MDXComponents } from "mdx/types";
+import { mdxComponents } from "@/components/blogComponents";
+
+export function useMDXComponents(components: MDXComponents): MDXComponents {
+	return { ...mdxComponents, ...components };
+}