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.spec.ts

@@ -35,6 +35,17 @@ describe('path-jail', () => {
     await expect(assertPathUnderRoot(tempRoot, allowedPath, ['server'])).resolves.toBe(allowedPath);
   });
 
+  it('accepts diagnostics log paths under diagnostics', async () => {
+    const diagnosticsDir = path.join(tempRoot, 'diagnostics');
+
+    fs.mkdirSync(diagnosticsDir, { recursive: true });
+    const logPath = path.join(diagnosticsDir, 'perf-session.jsonl');
+
+    fs.writeFileSync(logPath, '{}');
+
+    await expect(assertPathUnderRoot(tempRoot, logPath)).resolves.toBe(logPath);
+  });
+
   it('accepts cached plugin bundle paths under plugin-bundles', async () => {
     const bundleDir = path.join(tempRoot, 'plugin-bundles', 'example.plugin', '1.0.0');