feat: support media controller on mac

[gjermundgaraba]

This is an alternative to #171 that takes a slightly broader approach. Instead of adding macOS support via a registry/init() pattern on top of the existing mpris package, this PR consolidates platform media control into a single mediactl package with explicit wiring.
The mpris code is moved rather than wrapped, dead aliases are removed, and a internal/playback domain layer is introduced so seek/position values use time.Duration throughout instead of raw microseconds.
The trade-off is a larger diff, but it eliminates the side-effect import pattern and gives both platforms a clean shared contract.

Side-note: a neat follow-up to this that I am planning is to also get the song/album art showing in the native media controller.

• Add macOS media key support (play/pause/next/prev/seek via Control Centre, Touch Bar, Bluetooth headphones)
• Consolidate MPRIS and macOS backends into a unified mediactl package, replacing the separate mpris package
• Introduce internal/playback domain types using time.Duration instead of raw microseconds
• Use explicit wiring instead of init()-based registry pattern
• Add tests for volume conversion, IPC seek, notifier attachment, and darwin service

Closes #70

[bjarneo]

I think I like your approach better than mine. I'll spin it up on my mac and give it a go.

[bjarneo]

Works well!