feat(widget): send multiple files per multimodal message

[bsreenivas11]

Summary

• useFileUpload holds PendingFile[] and the attach input is multiple.
• Composer sends fileIds and renders N pending chips / transcript attachments.
• Depends on the client fileIds PR (bharath/multi-file-upload-client).

Written by Cursor Grok 4.6, using Cursor

Test plan

• Attach one file — still works
• Attach several files, send, confirm chips and transcript
• Disable attach at conversation maxFiles
• Merge after the client PR

[cursor]

PR Summary

Medium Risk
Expands the multimodal send path and parallel upload lifecycle (orphan deletes, limits, send gating); behavior depends on the matching client fileIds API being deployed.

Overview
Users can attach several files to one composer message instead of a single attachment. The file picker is multiple, and each selection is uploaded in parallel with its own preview chip; send is blocked while any upload is in flight or any attachment is in an error state.

useFileUpload now tracks PendingFile[] (stable id per item), per-file abort/cleanup, and maxFiles against sent plus pending count. SheetActions sends all ready files in one sendMultimodalMessage call and clears them via markFilesAsSent.

Transcript and display types move from fileInput to fileInputs, and sendMultimodalMessage forwards fileIds to the conversation client (pairs with the separate client fileIds change).

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[giannagerton]

We should be able to remove fileInput and just have fileInputs here since this type isn't exported from the package.

[giannagerton]

Same here, let's see if we can remove fileInput in favor of just fileInputs

[giannagerton]

LGTM just a few comments to look at before merging