Five files each carried their own copy of the WideCharToMultiByte /
MultiByteToWideChar UTF-8 conversion (injection_panel, audio_overrides,
imgui_layer's to_utf8, main's harness widen, and vk_layer_setup's inline
form). Replace them all with coop::narrow / coop::widen from one header.
audio_panel's image_basename dropped its lossy `c & 0x7F` ASCII mask for
the proper narrow(), so a non-ASCII game exe name is no longer mangled
in log lines.