Screensaver, idle dimming, HDMI/CEC standby and wake, "still watching?" prompt, power-key behaviour

Epic: #113 · Milestone: MC7 — TV shell

Tasks

• Screensaver after N minutes idle: artwork slideshow from the library or a clock, at reduced frame rate (≤ 10 fps, CPU ≤ 2 %); any key exits.
• Idle dimming, then display off via HDMI (signal off or CEC Standby to the TV) after M minutes; wake on CEC power-on / any remote key / phone remote.
• Playback end → return to detail screen; music continues under the screensaver.
• Power key on remotes: sleep (display off, network up, idle clocks) vs shutdown vs reboot — configurable.
• power-idle measured with the screensaver active.

Acceptance

Rig: after the idle timeout the capture shows the screensaver, then no signal; a CEC key wakes it within 2 s; watts within budget.

Size: S · Depends on: #92, #98, #64 · Blocks: —