IPC calls to show & hide bar widgets

## Feature Description

The settings entries for bar widgets have a show/hide (eye icon) button. It would be great to have this functionality via IPC.
```
dms ipc call widget show music
dms ipc call widget hide music
```

## Use Case

I want to use this with niri events, specifically `WindowFocusChanged`, to dynamically show or hide widgets based on the title/app-id of the focused window.

## Compositor

Is this feature specific to one compositor?

- [ ] All compositors
- [x] niri
- [ ] Hyprland
- [ ] dwl (MangoWC)
- [ ] sway

## Proposed Solution

I have only recently tried niri and DMS, so, I'm not familiar with the code base.

## Alternatives/Existing Solutions

I don't think there is a solution for this right now. Please let me know if I've missed something.

The issue names the widget settings eye-button behavior, the `dms ipc call widget show|hide ` entry points, and niri’s `WindowFocusChanged` event. Start by locating the existing IPC and niri event paths; done means the commands toggle the named widget and support the stated focus-change use case, with relevant tests updated or added.

Written by the indexing model from the issue text.