Add Volume() to Player interface and mpvPlayer implementation to
expose current volume level. Poll volume periodically and render a
vertical bar next to the metadata display during playback. Update
UI state and tests to support the new volume indicator.
Implement mpv JSON IPC backend for non-blocking playback, streamed
metadata (media-title/icy-title), and runtime controls (pause, mute,
volume, next/prev). Extend Player interface and wire a two-stage TUI
that switches to a dedicated playback view with keyboard shortcuts and
a styled hint bar. Fallback to legacy player when mpv is unavailable.