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).

integration/README.md

@@ -85,7 +85,9 @@ integration/
     ├── 05-actions.spec.ts        # context-menu read/unread, delete, spam (inbox)
     ├── 06-shared-folders.spec.ts # delegated folder: appears + read/unread/delete/spam
     ├── 07-drafts.spec.ts         # multiple recipients, changed sender, continue-draft button
-    └── 08-shared-moves.spec.ts   # moving mail across own/shared and shared/shared
+    ├── 08-shared-moves.spec.ts   # moving mail across own/shared and shared/shared
+    ├── 09-live-counters.spec.ts  # live unified/All-Mail counters (login + shared)
+    └── 10-attachments.spec.ts    # cross-account attachment download from All Mail
 ```
 
 ## Findings surfaced by the suite
@@ -110,6 +112,10 @@ because the UI behaviour is currently incomplete. Worth a look:
   "Move to" submenu offers the shared folder, but clicking it is a no-op.
   Shared ⇆ shared (same owner) moves work. Pinned with `test.fail` in
   `08-shared-moves`.
+- **Cross-account attachments (fixed).** Blobs are account-scoped, so viewing/
+  downloading an attachment on an All-Mail message from another account 404'd
+  against the active account. The download/preview path now routes to the
+  message's owning client + accountId (`10-attachments`).
 
 ## How the tests work
 

integration/tests/10-attachments.spec.ts

@@ -0,0 +1,77 @@
+import { test, expect } from '@playwright/test';
+import { ACCOUNTS } from './helpers/config';
+import { sendMail } from './helpers/smtp';
+import { JmapClient } from './helpers/jmap';
+import {
+  login,
+  addAccount,
+  switchAccount,
+  seedSettings,
+  folderRow,
+  openFolder,
+  emailItem,
+  expectEmailVisible,
+  forceSync,
+} from './helpers/app';
+
+/**
+ * Attachments on a message that belongs to a *different* account, opened from
+ * the cross-account All-Mail view. Blobs are account-scoped, so downloading one
+ * must route to the owning account's client + accountId — otherwise it 404s
+ * against the active account (the reported bug).
+ */
+const { alice, bob } = ACCOUNTS;
+const ATT = { filename: 'report.bin', contentType: 'application/octet-stream', content: 'hello-attachment-content-12345' };
+
+test.describe('Cross-account attachments', () => {
+  test.beforeEach(async () => {
+    for (const a of [alice, bob]) {
+      const j = await JmapClient.connect(a.email, a.password);
+      await j.reset();
+    }
+  });
+
+  test('an attachment on another account\'s All-Mail message downloads correctly', async ({ page }) => {
+    const subject = `IT attach ${Date.now()}`;
+    // Deliver a message with an attachment to bob.
+    await sendMail({ from: bob.email, authPass: bob.password, to: bob.email, subject, body: 'see attachment', attachment: ATT });
+
+    // Cross-account All Mail + always download attachments (don't preview).
+    await seedSettings(page, {
+      enableUnifiedMailbox: true,
+      enableCrossAllView: true,
+      unifiedCrossAccount: true,
+      includeGroupInUnified: true,
+      mailAttachmentAction: 'download',
+    });
+
+    // Make alice the active account, with bob added, so bob's message is
+    // genuinely cross-account when opened.
+    await login(page, alice);
+    await addAccount(page, bob);
+    await switchAccount(page, alice.email);
+    await forceSync(page);
+
+    // Open the All-Mail view and bob's message.
+    await expect(folderRow(page, { name: '__cross_all__' }).first()).toBeVisible();
+    await openFolder(page, { name: '__cross_all__' });
+    await forceSync(page);
+    await expectEmailVisible(page, subject);
+    await emailItem(page, subject).first().click();
+
+    // The attachment chip is present; clicking it downloads the blob from bob's
+    // account (pre-fix this 404s against alice and no download fires).
+    const chip = page.locator(`[data-testid="attachment"][data-attachment-name="${ATT.filename}"]`).first();
+    await chip.waitFor({ state: 'visible', timeout: 15000 });
+
+    const [download] = await Promise.all([
+      page.waitForEvent('download', { timeout: 15000 }),
+      chip.click(),
+    ]);
+
+    const stream = await download.createReadStream();
+    const chunks: Buffer[] = [];
+    for await (const c of stream) chunks.push(c as Buffer);
+    expect(Buffer.concat(chunks).toString()).toContain(ATT.content);
+  });
+});

integration/tests/helpers/smtp.ts

@@ -24,6 +24,8 @@ interface SendOptions {
   body: string;
   /** Extra headers (e.g. custom Message-ID / In-Reply-To for threading). */
   headers?: Record<string, string>;
+  /** Optional single attachment (sent as multipart/mixed, base64). */
+  attachment?: { filename: string; contentType: string; content: string };
 }
 
 class SmtpError extends Error {}
@@ -110,14 +112,38 @@ export async function sendMail(opts: SendOptions): Promise<void> {
       From: opts.from,
       To: recipients.join(', '),
       Subject: opts.subject,
-      'Content-Type': 'text/plain; charset=utf-8',
       ...opts.headers,
     };
+
+    let mime: string;
+    if (opts.attachment) {
+      const boundary = 'itmixed_boundary_0001';
+      headers['MIME-Version'] = '1.0';
+      headers['Content-Type'] = `multipart/mixed; boundary="${boundary}"`;
+      const b64 = Buffer.from(opts.attachment.content).toString('base64').replace(/(.{76})/g, '$1\r\n');
+      mime = [
+        `--${boundary}`,
+        'Content-Type: text/plain; charset=utf-8',
+        '',
+        crlf(opts.body),
+        `--${boundary}`,
+        `Content-Type: ${opts.attachment.contentType}; name="${opts.attachment.filename}"`,
+        `Content-Disposition: attachment; filename="${opts.attachment.filename}"`,
+        'Content-Transfer-Encoding: base64',
+        '',
+        b64,
+        `--${boundary}--`,
+      ].join('\r\n');
+    } else {
+      headers['Content-Type'] = 'text/plain; charset=utf-8';
+      mime = crlf(opts.body);
+    }
+
     const headerBlock = Object.entries(headers)
       .map(([k, v]) => `${k}: ${v}`)
       .join('\r\n');
     // Dot-stuff any line that begins with '.'
-    const safeBody = crlf(opts.body).replace(/\r\n\./g, '\r\n..');
+    const safeBody = mime.replace(/\r\n\./g, '\r\n..');
     send(`${headerBlock}\r\n\r\n${safeBody}\r\n.`);
     await waitReply('250');
     send('QUIT');