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/path-jail.ts

@@ -9,7 +9,8 @@ export const DEFAULT_USER_DATA_SUBDIRS = [
   'plugin-bundles',
   'plugin-cache',
   'themes',
-  'metoyou'
+  'metoyou',
+  'diagnostics'
 ] as const;
 
 export function isPathInside(parentPath: string, candidatePath: string): boolean {