Blobs are scoped per JMAP account, but the attachment download/preview path
always used the active account's client and accountId. Opening a message from a
different account in the unified / All-Mail view and downloading (or previewing)
an attachment therefore 404'd against the active account.
Route the blob fetch to the message's source instead:
- resolveBlobSource() picks the owning login's client
(getClientForAccount(sourceClientAccountId)) and the owner accountId
(sourceAccountId) for delegated/shared blobs, in the unified view;
- handleDownloadAttachment + the attachment-preview handlers use it;
- downloadBlob / fetchBlobAsObjectUrl / fetchBlobArrayBuffer gain an accountId
param (getBlobDownloadUrl/fetchBlob already had one).
Adds 10-attachments: an attachment on another account's All-Mail message
downloads with the correct bytes (verified to fail without the routing).
Stefan Hildebrandt
26c3d07d56