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-capture.ts

@@ -0,0 +1,80 @@
+import type { BrowserWindow } from 'electron';
+import type { AppMetricsSnapshot } from '../app-metrics';
+import { buildHighMemoryDiagnosticPayload } from './high-memory-snapshot.rules';
+import { collectImmediateRendererSamples } from './immediate-renderer-samples.collector';
+import { collectSessionContext } from './session-context.collector';
+import type { HighMemoryAlertRecord } from './high-memory-alert.store';
+import type { PerfDiagEntry } from './diagnostics.models';
+import { PerfDiagWriter } from './diagnostics.writer';
+
+export type HighMemoryCaptureReason = 'manual' | 'threshold';
+
+export interface CaptureHighMemoryDiagnosticsInput {
+  userDataPath: string;
+  sessionStartedAt: number;
+  metrics: AppMetricsSnapshot;
+  totalWorkingSetKb: number;
+  writer: PerfDiagWriter | null;
+  mainWindow: BrowserWindow | null;
+  reason: HighMemoryCaptureReason;
+}
+
+export async function captureHighMemoryDiagnostics(
+  input: CaptureHighMemoryDiagnosticsInput
+): Promise<HighMemoryAlertRecord> {
+  const detectedAt = Date.now();
+  const writer = input.writer ?? new PerfDiagWriter({
+    userDataPath: input.userDataPath,
+    sessionId: `${input.reason}-${detectedAt.toString(36)}-${process.pid}`
+  });
+  const immediateRendererEntries = await collectImmediateRendererSamples(input.mainWindow);
+  const environment = collectSessionContext({
+    sessionStartedAt: input.sessionStartedAt,
+    userDataPath: input.userDataPath
+  });
+
+  appendEntries(writer, immediateRendererEntries);
+  appendEntries(writer, [
+    {
+      collectedAt: detectedAt,
+      source: 'main',
+      type: 'environment',
+      payload: {
+        ...environment
+      }
+    },
+    {
+      collectedAt: detectedAt,
+      source: 'main',
+      type: 'high-memory',
+      payload: buildHighMemoryDiagnosticPayload({
+        detectedAt,
+        totalWorkingSetKb: input.totalWorkingSetKb,
+        metrics: input.metrics,
+        environment,
+        mainProcessMemory: process.memoryUsage(),
+        ringEntries: writer.bufferedEntries,
+        immediateRendererEntries,
+        sessionId: writer.sessionId
+      })
+    }
+  ]);
+
+  await writer.flushSnapshot(
+    input.reason === 'manual' ? 'manual-export' : 'high-memory-threshold'
+  );
+
+  return {
+    logFilePath: writer.snapshotFilePath,
+    detectedAt,
+    peakWorkingSetKb: input.totalWorkingSetKb,
+    sessionId: writer.sessionId,
+    reason: input.reason
+  };
+}
+
+function appendEntries(writer: PerfDiagWriter, entries: readonly PerfDiagEntry[]): void {
+  for (const entry of entries) {
+    writer.append(entry);
+  }
+}