feat: Show the time left on the sleep timer beside the playback controls

[abusch]

Shows the time left on the selected group's sleep timer on the main screen, to the right of the playback controls:

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• It only shows while a timer is set, counts down live, and uses the colour of the play modes that are on.
• It's drawn beside the controls rather than as part of them, so they stay centred and the volume slider keeps its width. It's left out when there is no room, e.g. with the group panel open in a narrow terminal.
• The screen now redraws while any timer is running, not just while the playback menu is open.
• The menu and the main screen share sleep_timer_text to format the time left.

Tried on a real speaker: set a 15-minute timer, checked the main screen, then cancelled it.

🤖 Generated with Claude Code