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/ipc/system.ts

@@ -68,6 +68,7 @@ import {
   grantPluginReadRoot,
   resolveReadablePath
 } from '../path-jail';
+import { isReadableRegularFile } from './file-read.rules';
 
 const DEFAULT_MIME_TYPE = 'application/octet-stream';
 const MAX_ACTIVE_DESKTOP_NOTIFICATIONS = 20;
@@ -654,9 +655,19 @@ export function setupSystemHandlers(): void {
       return null;
     }
 
-    const data = await fsp.readFile(scopedPath);
+    try {
+      const stats = await fsp.stat(scopedPath);
 
-    return data.toString('base64');
+      if (!isReadableRegularFile(stats)) {
+        return null;
+      }
+
+      const data = await fsp.readFile(scopedPath);
+
+      return data.toString('base64');
+    } catch {
+      return null;
+    }
   });
 
   ipcMain.handle('read-file-chunk', async (_event, filePath: string, start: number, end: number) => {
@@ -666,17 +677,27 @@ export function setupSystemHandlers(): void {
       return null;
     }
 
-    const fileHandle = await fsp.open(scopedPath, 'r');
-
     try {
-      const safeStart = Math.max(0, Math.trunc(start));
-      const safeEnd = Math.max(safeStart, Math.trunc(end));
-      const buffer = Buffer.alloc(safeEnd - safeStart);
-      const result = await fileHandle.read(buffer, 0, buffer.length, safeStart);
+      const stats = await fsp.stat(scopedPath);
 
-      return buffer.subarray(0, result.bytesRead).toString('base64');
-    } finally {
-      await fileHandle.close();
+      if (!isReadableRegularFile(stats)) {
+        return null;
+      }
+
+      const fileHandle = await fsp.open(scopedPath, 'r');
+
+      try {
+        const safeStart = Math.max(0, Math.trunc(start));
+        const safeEnd = Math.max(safeStart, Math.trunc(end));
+        const buffer = Buffer.alloc(safeEnd - safeStart);
+        const result = await fileHandle.read(buffer, 0, buffer.length, safeStart);
+
+        return buffer.subarray(0, result.bytesRead).toString('base64');
+      } finally {
+        await fileHandle.close();
+      }
+    } catch {
+      return null;
     }
   });
 
@@ -728,6 +749,17 @@ export function setupSystemHandlers(): void {
     return true;
   });
 
+  ipcMain.handle('append-file-bytes', async (_event, filePath: string, bytes: Uint8Array) => {
+    const scopedPath = await resolveWritableUserDataFilePath(filePath);
+
+    if (!scopedPath) {
+      return false;
+    }
+
+    await fsp.appendFile(scopedPath, Buffer.from(bytes));
+    return true;
+  });
+
   ipcMain.handle('delete-file', async (_event, filePath: string) => {
     const scopedPath = await resolveWritableUserDataFilePath(filePath);