Add basic support to sync directories with a GUI to enter sync paths and view existing file syncs. Users must enter paths manually; no GUI at this stage.

Not in scope

• Metadata about existing syncs like status, size, etc
• GUI for picking folders

Dependencies

• Build a mutagen API client for coder-desktop-macos internal#379
• Agent SSH server should use a consistent key over workspace restarts coder#16490
• Bundle Mutagen binary in App Bundle #60
• manage mutagen daemon lifecycle for coder-desktop-macos internal#381