fix: prevent Enter key from submitting during file upload

[the-quantum-nargle]

This PR fixes an issue where pressing Enter would unintentionally submit a message even if a file was still uploading.

Now, the Enter key is disabled while files are uploading in the multimodal input component, matching the behavior of the disabled Send button

Changes:

• Added a check in the onKeyDown handler to prevent submission if uploadQueue.length > 0.
• Shows a toast error if attempted.
• This improves UX and prevents incomplete messages from being sent while uploads are in progress.

[vercel]

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[the-quantum-nargle]

Hi @jeremyphilemon
Would you be able to take a look at this PR when you get a chance? Let me know if anything needs adjusting. Thanks!