fix: move fastembed import to the isnode condition check

[antpb]

In current state it looks like we had a bit of a regression importing fastembed at the top level again, which causes build failures in non-Node environments due to native dependencies (.node files) like Cloudflare Workers and other edge workers.

This PR:

• Removes top-level import of EmbeddingModel from fastembed
• Dynamically imports fastembed package only when local embeddings are needed
• Ensures native modules are only loaded in Node.js environment

This allows the package to be built for both browser and Node.js environments while maintaining all existing functionality.

Testing:

• Confirm local embeddings still work in Node.js
• Verify builds succeed in browser/Cloudflare Worker environments (done!)
• Maintained original behavior for remote embedding fallbacks

[augchan42]

Sorry, that change was from me. Now I understand why it's needed.

[shakkernerd]

Great!