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/hooks/use-file-explorer-actions.ts

@@ -1,5 +1,6 @@
 import { useCallback, useMemo } from "react";
 import { useSessionStore, type AgentFileExplorerState } from "@/stores/session-store";
+import { explorerFileFromReadResult } from "@/file-explorer/read-result";
 
 function createExplorerState(): AgentFileExplorerState {
   return {
@@ -130,26 +131,20 @@ export function useFileExplorerActions(params: { serverId: string } & FileExplor
       }
 
       try {
-        const payload = await client.exploreFileSystem(
-          normalizedWorkspaceRoot,
-          normalizedPath,
-          "list",
-        );
+        const directory = await client.listDirectory(normalizedWorkspaceRoot, normalizedPath);
         updateExplorerState((state) => {
           const nextState: AgentFileExplorerState = {
             ...state,
             isLoading: false,
-            lastError: payload.error ?? null,
+            lastError: null,
             pendingRequest: null,
             directories: state.directories,
             files: state.files,
           };
 
-          if (!payload.error && payload.directory) {
-            const directories = new Map(state.directories);
-            directories.set(payload.directory.path, payload.directory);
-            nextState.directories = directories;
-          }
+          const directories = new Map(state.directories);
+          directories.set(directory.path, directory);
+          nextState.directories = directories;
 
           return nextState;
         });
@@ -201,34 +196,29 @@ export function useFileExplorerActions(params: { serverId: string } & FileExplor
       }
 
       try {
-        const payload = await client.exploreFileSystem(
-          normalizedWorkspaceRoot,
-          normalizedPath,
-          "file",
-        );
+        const file = await client.readFile(normalizedWorkspaceRoot, normalizedPath);
         updateExplorerState((state) => {
           const nextState: AgentFileExplorerState = {
             ...state,
             isLoading: false,
+            lastError: null,
             pendingRequest: null,
             directories: state.directories,
             files: state.files,
           };
 
-          if (!payload.error && payload.file) {
-            const files = new Map(state.files);
-            files.set(payload.file.path, payload.file);
-            nextState.files = files;
-          } else if (payload.error) {
-            nextState.lastError = payload.error;
-          }
+          const files = new Map(state.files);
+          const explorerFile = explorerFileFromReadResult(file);
+          files.set(explorerFile.path, explorerFile);
+          nextState.files = files;
 
           return nextState;
         });
-      } catch {
+      } catch (error) {
         updateExplorerState((state) => ({
           ...state,
           isLoading: false,
+          lastError: error instanceof Error ? error.message : "Failed to load file preview",
           pendingRequest: null,
         }));
       }