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/file-explorer/read-result.ts

@@ -0,0 +1,15 @@
+import type { FileReadResult } from "@server/client/daemon-client";
+import type { ExplorerFile } from "@/stores/session-store";
+
+export function explorerFileFromReadResult(file: FileReadResult): ExplorerFile {
+  const isText = file.kind === "text";
+  return {
+    path: file.path,
+    kind: file.kind,
+    encoding: isText ? "utf-8" : "none",
+    content: isText ? new TextDecoder().decode(file.bytes) : undefined,
+    mimeType: file.mime,
+    size: file.size,
+    modifiedAt: file.modifiedAt,
+  };
+}