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.ts

@@ -0,0 +1,6 @@
+import type { Stats } from 'fs';
+
+/** Only regular files can be read through the read-file IPC surface. */
+export function isReadableRegularFile(stats: Pick<Stats, 'isFile'>): boolean {
+  return stats.isFile();
+}