Improve attachment memory safety, downloads, and high-memory alert UX.

Stream large receives to disk with chunk acks to cap renderer RAM, evict
off-screen display blobs, and route exports through a disk-aware download
service. Fix the high-memory dialog (backdrop dismiss, copy, log actions),
allow diagnostics paths in the path jail, and restore persisted image
hydration after reload.

Co-authored-by: Cursor <cursoragent@cursor.com>

electron/diagnostics/high-memory-capture.spec.ts

@@ -0,0 +1,57 @@
+import {
+  beforeEach,
+  describe,
+  expect,
+  it,
+  vi
+} from 'vitest';
+import * as os from 'os';
+import * as path from 'path';
+import * as fsp from 'fs/promises';
+
+import { captureHighMemoryDiagnostics } from './high-memory-capture';
+
+vi.mock('./immediate-renderer-samples.collector', () => ({
+  collectImmediateRendererSamples: vi.fn(async () => [])
+}));
+
+vi.mock('./session-context.collector', () => ({
+  collectSessionContext: vi.fn(() => ({
+    platform: 'linux',
+    userDataPath: '/tmp/user-data'
+  }))
+}));
+
+describe('captureHighMemoryDiagnostics', () => {
+  let userDataPath = '';
+
+  beforeEach(async () => {
+    userDataPath = await fsp.mkdtemp(path.join(os.tmpdir(), 'metoyou-high-memory-capture-'));
+  });
+
+  it('writes a diagnostics snapshot and returns an alert record', async () => {
+    const record = await captureHighMemoryDiagnostics({
+      userDataPath,
+      sessionStartedAt: Date.now() - 60_000,
+      metrics: {
+        collectedAt: Date.now(),
+        processes: [
+          {
+            pid: 1,
+            type: 'Browser',
+            workingSetKb: 2_200_000
+          }
+        ]
+      },
+      totalWorkingSetKb: 2_200_000,
+      writer: null,
+      mainWindow: null,
+      reason: 'manual'
+    });
+
+    expect(record.peakWorkingSetKb).toBe(2_200_000);
+    expect(record.reason).toBe('manual');
+    expect(record.logFilePath).toContain(userDataPath);
+    await expect(fsp.stat(record.logFilePath)).resolves.toBeDefined();
+  });
+});