feat: stream files as binary frames over WebSocket (#659)

Replace base64-in-JSON file transfers with binary WebSocket frames to
unblock the JS thread when assistant markdown messages contain images.
Receive cost (JSON.parse over multi-MB base64) and persist cost (base64
re-encode to disk) both removed.

- Split daemon-client `exploreFileSystem` into `listDirectory` + `readFile`;
  `readFile` returns `{ bytes, mime, size, modifiedAt }`. Encoding stays
  private to the seam.
- New `packages/server/src/shared/binary-frames/` module hosts terminal
  frames (moved from `terminal-stream-protocol.ts`) and new file-transfer
  opcodes (`FileBegin` / `FileChunk` / `FileEnd`) with request-id correlation.
- Add optional `acceptBinary` to `FileExplorerRequestSchema`. New daemons
  emit binary frames when set; legacy JSON path remains for old clients.
  Compat fallback lives only inside `readFile`.
- New `persistAttachmentFromBytes` writes bytes directly: native via
  `expo-file-system` `File.write(Uint8Array)`, web via Blob -> IndexedDB,
  desktop via new `write_attachment_bytes` IPC. `persistAttachmentFromBase64`
  removed.

packages/app/src/desktop/attachments/desktop-preview-url.ts

@@ -1,14 +1,10 @@
 import type { AttachmentMetadata } from "@/attachments/types";
 import { fileUriToPath } from "@/attachments/utils";
 import { invokeDesktopCommand } from "@/desktop/electron/invoke";
+import { Buffer } from "buffer";
 
 function base64ToUint8Array(base64: string): Uint8Array {
-  const binary = atob(base64);
-  const bytes = new Uint8Array(binary.length);
-  for (let i = 0; i < binary.length; i += 1) {
-    bytes[i] = binary.charCodeAt(i);
-  }
-  return bytes;
+  return new Uint8Array(Buffer.from(base64, "base64"));
 }
 
 const activeObjectUrls = new Set<string>();