fix(blocks): resolve the Create preview iframe under the Pages basePath

[yyyyaaa]

Summary

On the deployed site, /blocks/blocks/create/ embedded /blocks/create/preview/ — the basePath was missing — so GitHub Pages served the 404 page and the docs shell rendered inside itself (the "inception" screenshot).

• Root cause: BLOCKS_PAGES is a plain env var, so it is only defined for server code. withBase() runs in the browser for the preview iframe (preview-frame.tsx), where BASE_PATH evaluated to ''.
• Fix: next.config.ts inlines a NEXT_PUBLIC_BLOCKS_PAGES mirror into the client bundle and site.ts checks both variables.

Verified against a real pnpm build:pages export served under /blocks/: clicking Create yields iframe src="/blocks/blocks/create/preview/?d=…", the preview renders the platform wall, no nested frame, no 4xx responses.

Generated with Devin

[tenki-reviewer]

Review complete. No issues found — approved ✅.

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This small PR (2 files, ~30 diff lines) touches the blocks documentation app's Next.js config and its shared site library. The scan passed the changed files through correctness and holistic review with no findings above threshold, so no inline comments were produced and no defects were flagged for the author.

Files	Change
apps/blocks/next.config.ts	Adjusts Next.js build/runtime configuration for the blocks docs app.
apps/blocks/src/lib/site.ts	Updates shared site helper logic used by the docs app.

_{Reviewed commit: adf0ea5}