Add Sleep Timer support (set_sleep_timer endpoint)

[spacenono]

Implements #1659.

The Spotify mobile app's Sleep timer (Now Playing → ⋯ → Sleep timer) sends a
set_sleep_timer command that librespot currently rejects with
"Not implemented { unknown endpoint: set_sleep_timer }". This PR handles it.

Behaviour

• A duration timer arms a one-shot countdown; playback is paused when it
  expires, and the paused state is propagated back to the controlling client.
• A zero/missing duration disarms the timer; re-sending a timer re-arms it.
• A manual resume does not cancel the timer (it still fires at its deadline),
  matching the official client.

Design

• A single absolute deadline (Option<tokio::time::Instant>) is stored on the
  task and awaited with sleep_until inside the existing select! loop, so
  recreating the future every iteration never drifts the deadline (same spirit
  as the keep-alive timer in Rework session keep-alive logic v2 #1359, without pinning a Sleep).
• The timer_type discriminator is kept as a raw string so unsupported or
  future types are logged by name.

Scope / future work

• duration is fully implemented and tested end-to-end on an iPhone against a
  macOS build (timer armed, fired, paused, state propagated).
• timestamp is parsed defensively (no real-world sample captured yet).
• The end of track option sends type: "end_of_track"; it is a different,
  event-driven mechanism and is logged + ignored for now.

Known limitation — open question for maintainers

With this change the device honours the command and pauses correctly, but the
mobile app still briefly shows a "you can't use the sleep timer on this device"
popup. This appears to be client-side capability gating for the (recently
added, Aug 2025) cross-device sleep timer: the device never advertises the
corresponding capability. The most likely place is the empty
Capabilities.connect_capabilities (tag 27) string field, but I couldn't find
the expected token/format in any public source. Do you know how this
capability should be advertised? Happy to add it if so.

Breaking change

Adds a SetSleepTimer variant to the public, non-exhaustive
core::dealer::protocol::Command enum — downstream exhaustive matches must be
updated.

[spacenono]

Note on the fix(metadata) commit in this PR

Heads-up so this doesn't look out of scope: CI runs clippy with -D warnings, and clippy 1.96 (released 2026-05-25) introduced a new lint, clippy::unnecessary_sort_by, which flags pre-existing code in metadata/src/audio/item.rs:197 — unrelated to the sleep timer. This PR just happened to be the first to run CI against the newer clippy, so it surfaced the warning here.

I added a separate, clearly-scoped commit (fix(metadata): satisfy clippy::unnecessary_sort_by) that applies clippy's own suggestion (sort_by_key + Reverse, no behaviour change) so the pipeline goes green.

Happy to drop that commit or move it to its own PR if you'd rather handle the cleanup separately — just let me know.