The Files page UI sat at 0% throughout an upload because uploadBlob()
uses fetch(), which does not surface upload progress events. The store
set loaded=0 before the call and loaded=file.size after it, so users
saw the progress bar jump from 0% straight to 100% on completion --
and on slow connections (or large files) it appeared frozen.
Switch uploadBlob() to XHR when the caller passes onProgress or an
AbortSignal, so progress events from xhr.upload.onprogress can drive
the UI. Callers that don't pass either keep the fetch path so we
preserve the existing 401-retry behaviour in authenticatedFetch().
Wire the file store to pass both onProgress (updates uploadProgress
in real time) and the existing AbortController's signal (so cancel
now actually aborts the network request, not just the post-upload
createFileNode step).
uploadBlob() is part of IJMAPClient so the signature change is also
applied to the demo client (synthesises 0% then 100%).
