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/desktop/attachments/desktop-attachment-store.test.ts

@@ -4,6 +4,7 @@ import { createDesktopAttachmentStore } from "./desktop-attachment-store";
 const {
   copyDesktopAttachmentFileMock,
   writeDesktopAttachmentBase64Mock,
+  writeDesktopAttachmentBytesMock,
   deleteDesktopAttachmentFileMock,
   garbageCollectDesktopAttachmentFilesMock,
   readDesktopFileBase64Mock,
@@ -18,6 +19,10 @@ const {
     path: "/managed/att_2.png",
     byteSize: 4,
   })),
+  writeDesktopAttachmentBytesMock: vi.fn(async () => ({
+    path: "/managed/att_bytes.png",
+    byteSize: 4,
+  })),
   deleteDesktopAttachmentFileMock: vi.fn(async () => true),
   garbageCollectDesktopAttachmentFilesMock: vi.fn(async () => 0),
   readDesktopFileBase64Mock: vi.fn(async () => "AAECAw=="),
@@ -28,6 +33,7 @@ const {
 vi.mock("./desktop-file-commands", () => ({
   copyDesktopAttachmentFile: copyDesktopAttachmentFileMock,
   writeDesktopAttachmentBase64: writeDesktopAttachmentBase64Mock,
+  writeDesktopAttachmentBytes: writeDesktopAttachmentBytesMock,
   deleteDesktopAttachmentFile: deleteDesktopAttachmentFileMock,
   garbageCollectDesktopAttachmentFiles: garbageCollectDesktopAttachmentFilesMock,
 }));
@@ -42,6 +48,7 @@ describe("desktop attachment store", () => {
   beforeEach(() => {
     copyDesktopAttachmentFileMock.mockClear();
     writeDesktopAttachmentBase64Mock.mockClear();
+    writeDesktopAttachmentBytesMock.mockClear();
     deleteDesktopAttachmentFileMock.mockClear();
     garbageCollectDesktopAttachmentFilesMock.mockClear();
     readDesktopFileBase64Mock.mockClear();
@@ -86,28 +93,30 @@ describe("desktop attachment store", () => {
     });
   });
 
-  it("saves raw base64 sources via desktop filesystem writes", async () => {
+  it("saves raw byte sources via desktop filesystem writes", async () => {
     const store = createDesktopAttachmentStore();
+    const bytes = new Uint8Array([0, 1, 2, 3]);
     const attachment = await store.save({
-      id: "att_base64",
+      id: "att_bytes",
       mimeType: "image/png",
       fileName: "inline.png",
       source: {
-        kind: "base64",
-        base64: "AAECAw==",
+        kind: "bytes",
+        bytes,
       },
     });
 
-    expect(writeDesktopAttachmentBase64Mock).toHaveBeenCalledWith({
-      attachmentId: "att_base64",
-      base64: "AAECAw==",
+    expect(writeDesktopAttachmentBytesMock).toHaveBeenCalledWith({
+      attachmentId: "att_bytes",
+      bytes,
       extension: ".png",
     });
+    expect(writeDesktopAttachmentBase64Mock).not.toHaveBeenCalled();
     expect(attachment).toMatchObject({
-      id: "att_base64",
+      id: "att_bytes",
       mimeType: "image/png",
       storageType: "desktop-file",
-      storageKey: "/managed/att_2.png",
+      storageKey: "/managed/att_bytes.png",
       fileName: "inline.png",
       byteSize: 4,
     });