feat: multi-monitor playback and per-display modes

[tharropoulos]

• closes Add commandline to specify output (wayland) #25
• closes render wallpaper on every connected display #14
• add phonto displays subcommand that prints detected outputs with the native id to use on each os
• mirror a single video across every connected display by default on both macos and wayland, sharing one decoder per backend
• add --display ID PATH and --display-rand ID cli flags (repeatable) plus matching [[display]] config blocks for pinning videos per display
• add [[alias]] config blocks that map a portable name to per-os native ids so one dotfile works on both macos and wayland
• attach new monitors automatically on hot-plug, drop surfaces cleanly on unplug, recover from hdmi cycles without restart
• expand leading ~/ in every config and cli path field so the same path works across /Users and /home
• refactor Backend::run to take a typed Playback enum (mirror or per-display) and split wayland's GlRenderer into a shared gl context plus per-output OutputRender
• spawn one decoder thread per assignment on wayland (mirror collapses to one), build one AVPlayer per assignment on macos with a shared battery observer
  driving them all