[Feature] mpris widget: add a seek bar (and maybe shuffle/loop/volume)

the mpris widget only has play/pause and next/prev right now. would be really nice to get a seek/progress bar so you can see position and scrub through a track. shuffle, loop and per-player volume would be great too since the Player interface already exposes them.

happy to open a PR if you're up for it, just lmk if there's an approach you'd prefer.

Start by reading the mpris widget and the Player interface, especially the existing play/pause and next/previous controls and the exposed shuffle, loop, and volume operations. Clarify the intended scope, then verify that the widget shows track progress and supports scrubbing; the optional controls should work consistently if included.

Written by the indexing model from the issue text.