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/index.ts

@@ -15,8 +15,11 @@ export {
   attachRendererDiagnosticsHooks,
   ensurePerfDiagIpcRegistered,
   getActivePerfDiagWriter,
+  HIGH_MEMORY_ALERT_PENDING_CHANNEL,
   isPerfDiagActive,
+  shutdownHighMemoryMonitoring,
   shutdownPerfDiagnostics,
+  startHighMemoryMonitoring,
   startPerfDiagnostics
 } from './diagnostics.lifecycle';
 export type { PerfDiagEntry, PerfDiagEntryType, PerfDiagSource } from './diagnostics.models';