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/desktop/src/features/attachments.ts

@@ -90,6 +90,37 @@ export async function writeAttachmentBase64(input: {
   };
 }
 
+function normalizeBytes(value: unknown): Uint8Array {
+  if (value instanceof Uint8Array) {
+    return value;
+  }
+  if (value instanceof ArrayBuffer) {
+    return new Uint8Array(value);
+  }
+  if (Array.isArray(value)) {
+    return Uint8Array.from(value);
+  }
+  throw new Error("Attachment byte payload is required.");
+}
+
+export async function writeAttachmentBytes(input: {
+  attachmentId?: unknown;
+  bytes?: unknown;
+  extension?: unknown;
+}): Promise<AttachmentFileResult> {
+  const bytes = normalizeBytes(input.bytes);
+  const targetPath = await buildManagedAttachmentPath({
+    attachmentId: input.attachmentId,
+    extension: input.extension,
+  });
+  await writeFile(targetPath, bytes);
+  const fileInfo = await stat(targetPath);
+  return {
+    path: targetPath,
+    byteSize: fileInfo.size,
+  };
+}
+
 export async function copyAttachmentFileToManagedStorage(input: {
   attachmentId?: unknown;
   sourcePath?: unknown;