Serialize Spotify remote skip commands

[jsgrrchg]

Summary

• serialize Spotify remote next/previous commands through a FIFO queue
• allow only one remote skip request in flight at a time
• assign operation IDs and ignore stale results or results from a previously active device
• keep skip results separate from the shared playback result receiver
• refresh remote playback state after each accepted operation completes

Why

Rapid next/previous input could launch overlapping Spotify Web API requests. Network timing could reorder those requests, while replacing the shared result receiver caused earlier outcomes to be dropped locally even though Spotify could still apply them.

The queue establishes a single ownership boundary for remote skip operations and preserves input order without duplicating Spotify command error handling.

Testing

• cargo fmt --check
• cargo test (242 passed)
• cargo clippy --all-targets
• git diff --check

Fixes #263

[jsgrrchg]

Residual risk, Spotify API behavior under real-world latency, token expiration, and device changes is covered by simulated states, not live integration tests. Not a blocker.