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-alert.store.ts

@@ -1,11 +1,14 @@
 import * as fsp from 'fs/promises';
 import * as path from 'path';
 
+export type HighMemoryAlertReason = 'manual' | 'threshold';
+
 export interface HighMemoryAlertRecord {
   logFilePath: string;
   detectedAt: number;
   peakWorkingSetKb: number;
   sessionId: string;
+  reason?: HighMemoryAlertReason;
 }
 
 export function resolveHighMemoryAlertPath(userDataPath: string): string {
@@ -31,7 +34,10 @@ export async function readHighMemoryAlert(userDataPath: string): Promise<HighMem
       logFilePath: parsed.logFilePath,
       detectedAt: parsed.detectedAt,
       peakWorkingSetKb: parsed.peakWorkingSetKb,
-      sessionId: parsed.sessionId
+      sessionId: parsed.sessionId,
+      ...(parsed.reason === 'manual' || parsed.reason === 'threshold'
+        ? { reason: parsed.reason }
+        : {})
     };
   } catch {
     return null;