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/daemon/daemon-manager.ts

@@ -10,6 +10,7 @@ import {
   garbageCollectManagedAttachmentFiles,
   readManagedFileBase64,
   writeAttachmentBase64,
+  writeAttachmentBytes,
 } from "../features/attachments.js";
 import {
   checkForAppUpdate,
@@ -539,6 +540,7 @@ export function createDaemonCommandHandlers(): Record<string, DesktopCommandHand
     desktop_get_system_idle_time: () => powerMonitor.getSystemIdleTime() * 1000,
     cli_daemon_status: () => getCliDaemonStatus(),
     write_attachment_base64: (args) => writeAttachmentBase64(args ?? {}),
+    write_attachment_bytes: (args) => writeAttachmentBytes(args ?? {}),
     copy_attachment_file: (args) => copyAttachmentFileToManagedStorage(args ?? {}),
     read_file_base64: (args) => readManagedFileBase64(args ?? {}),
     delete_attachment_file: (args) => deleteManagedAttachmentFile(args ?? {}),