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/ipc/file-read.rules.spec.ts

@@ -0,0 +1,16 @@
+import {
+  describe,
+  expect,
+  it
+} from 'vitest';
+import { isReadableRegularFile } from './file-read.rules';
+
+describe('file-read.rules', () => {
+  it('accepts regular files', () => {
+    expect(isReadableRegularFile({ isFile: () => true })).toBe(true);
+  });
+
+  it('rejects directories and other non-file paths', () => {
+    expect(isReadableRegularFile({ isFile: () => false })).toBe(false);
+  });
+});