fix(attachments): download/view attachments on cross-account All-Mail messages

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).

components/email/email-viewer.tsx

@@ -3704,6 +3704,8 @@ export function EmailViewer({
                         )}
                         title={`${opensPreview ? tFiles('preview') : t('download')} ${getAttachmentDisplayName(attachment.name, attachment.type)}`}
                         onClick={() => handleEffectiveAttachmentOpen(attachment)}
+                        data-testid="attachment"
+                        data-attachment-name={attachment.name}
                         draggable={dragProps.draggable}
                         onPointerEnter={dragProps.onPointerEnter}
                         onDragStart={dragProps.onDragStart}
@@ -4478,6 +4480,8 @@ export function EmailViewer({
                   )}
                   title={`${opensPreview ? tFiles('preview') : t('download')} ${getAttachmentDisplayName(attachment.name, attachment.type)}`}
                   onClick={() => handleEffectiveAttachmentOpen(attachment)}
+                  data-testid="attachment"
+                  data-attachment-name={attachment.name}
                   draggable={dragProps.draggable}
                   onPointerEnter={dragProps.onPointerEnter}
                   onDragStart={dragProps.onDragStart}
@@ -4622,6 +4626,8 @@ export function EmailViewer({
                     )}
                     title={`${opensPreview ? tFiles('preview') : t('download')} ${getAttachmentDisplayName(attachment.name, attachment.type)}`}
                     onClick={() => handleEffectiveAttachmentOpen(attachment)}
+                    data-testid="attachment"
+                    data-attachment-name={attachment.name}
                     draggable={dragProps.draggable}
                     onPointerEnter={dragProps.onPointerEnter}
                     onDragStart={dragProps.onDragStart}