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/server/src/terminal/terminal-session-controller.ts

@@ -19,7 +19,7 @@ import {
   encodeTerminalSnapshotPayload,
   encodeTerminalStreamFrame,
   type TerminalStreamFrame,
-} from "../shared/terminal-stream-protocol.js";
+} from "../shared/binary-frames/index.js";
 import { TerminalOutputCoalescer } from "./terminal-output-coalescer.js";
 import type { TerminalSession } from "./terminal.js";
 import type { TerminalManager, TerminalsChangedEvent } from "./terminal-manager.js";