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/attachments/service.ts

@@ -31,8 +31,8 @@ export async function persistAttachmentFromDataUrl(input: {
   });
 }
 
-export async function persistAttachmentFromBase64(input: {
-  base64: string;
+export async function persistAttachmentFromBytes(input: {
+  bytes: Uint8Array;
   mimeType?: string;
   fileName?: string | null;
   id?: string;
@@ -42,7 +42,7 @@ export async function persistAttachmentFromBase64(input: {
     id: input.id,
     mimeType: input.mimeType,
     fileName: input.fileName,
-    source: { kind: "base64", base64: input.base64 },
+    source: { kind: "bytes", bytes: input.bytes },
   });
 }