fix(peer): restore terminal control and file downloads

[bobleer]

Summary

• keep peer download destinations on the controller and stream source bytes from the controlled device
• support recursive directory export instead of passing directories to std::fs::copy
• fan out SSH-backed terminal events, prioritize terminal RPCs, and forward SIGINT/SIGTSTP control bytes
• document split-endpoint download and terminal transport invariants

Root causes

The native save dialog runs on controller A, but export_local_file_to_path was HostInvoked on peer B. That made B attempt to write an A-local path and could return Windows access denied (os error 5). Directory downloads also reached std::fs::copy, which cannot copy a directory.

SSH-backed PTY events were emitted only to the peer desktop webview, terminal RPCs could sit behind two background requests, and remote terminal signals returned success without affecting the PTY.

Validation

• pnpm run type-check:web
• pnpm --dir src/web-ui run test:run src/infrastructure/api/adapters/peer-device-adapter.test.ts src/tools/file-system/services/workspaceFileTransfer.test.ts
• cargo check -p bitfun-desktop
• cargo test -p bitfun-desktop --lib remote_terminal_signal
• cargo test -p bitfun-desktop --lib copy_directory_recursive_copies_nested_binary_files
• git diff --check

Physical Windows to macOS peer smoke testing is still recommended before release.