Extend the cross-account blob routing beyond download/preview to every blob
fetch in the message viewer, so a message opened from a different account in the
unified / All-Mail view renders and exports correctly instead of 404ing against
the active account:
- inline cid: images, drag-to-desktop, attachment thumbnails, the "download all"
zip bundle, and the S/MIME / TNEF / embedded-rfc822 blob reads now use a
resolved blobClient (getClientForAccount(sourceClientAccountId)) and the owner
blobAccountId (sourceAccountId), computed once from the open message's source;
- fetchBlobAsObjectUrl / fetchBlobArrayBuffer / fetchBlob calls pass the
accountId (the client methods gained the param in the previous commit);
- non-cross-account behaviour is unchanged (blobClient === active client).
Extends 10-attachments with an inline-image case (verified to fall back to the
placeholder without the routing). The SMTP helper can now send multipart/related
inline images.