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).

lib/jmap/client-interface.ts

@@ -222,9 +222,9 @@ export interface IJMAPClient {
   ): Promise<{ blobId: string; size: number; type: string }>;
   getBlobDownloadUrl(blobId: string, name?: string, type?: string, accountId?: string): string;
   fetchBlob(blobId: string, name?: string, type?: string, accountId?: string): Promise<Blob>;
-  fetchBlobAsObjectUrl(blobId: string, name?: string, type?: string): Promise<string>;
-  fetchBlobArrayBuffer(blobId: string, name?: string, type?: string): Promise<ArrayBuffer>;
-  downloadBlob(blobId: string, name?: string, type?: string): Promise<void>;
+  fetchBlobAsObjectUrl(blobId: string, name?: string, type?: string, accountId?: string): Promise<string>;
+  fetchBlobArrayBuffer(blobId: string, name?: string, type?: string, accountId?: string): Promise<ArrayBuffer>;
+  downloadBlob(blobId: string, name?: string, type?: string, accountId?: string): Promise<void>;
 
   // ── Identities ────────────────────────────────────────────────
   getIdentities(): Promise<Identity[]>;