Add an optional MPRIS “Now Playing” item to Dock

### Problem

Right now, media controls in elementary are mainly available from the sound indicator in the top panel.
For frequent actions (play/pause, next/previous, checking what is playing), this adds extra clicks and pointer movement every time.

There is no persistent, glanceable media surface in the Dock itself, even though the Dock is the main interaction area for launching and switching apps.

### Proposal

Add an optional Now Playing Dock item that appears only when an MPRIS-compatible player is active (Playing or Paused), and hides automatically when no player is available.

Proposed behavior:

- Show artwork, track title, and artist.
- Provide media controls: previous, play/pause, next.

- Support two visual modes:
- Normal mode: full card in the Dock.
- Minimal mode: cover-only icon; controls/info shown in tooltip.

- Add right-click toggles for:
- Minimal Mode
- Cover Animation (subtle, optional)
- Seek Bar (optional)

- Keep the feature lightweight and non-intrusive:
- No Dock changes when no media is active.
- Persist user preferences via settings keys.
- Follow existing Dock styling and spacing.

I already implemented a working prototype in my fork, including screenshots and behavior details:
https://github.com/Juandamian18/Dock-with-Now-Playing

### Prior Art (Optional)