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.spec.ts

@@ -33,7 +33,8 @@ describe('high-memory-alert.store', () => {
       logFilePath: path.join(userDataPath, 'diagnostics', 'perf-session.jsonl'),
       detectedAt: 1_700_000_000_000,
       peakWorkingSetKb: 2_200_000,
-      sessionId: 'session-1'
+      sessionId: 'session-1',
+      reason: 'threshold' as const
     };
 
     await writeHighMemoryAlert(userDataPath, record);