Allow popping up the audio devices panel with a keybind

### Feature Description

Allow access to this panel with a keybind.

### Use Case

It would be great if I could access this panel with a keybind since I use it frequently to mute or adjust individual apps.

### Compositor(s)

- [ ] All compositors
- [x] Niri
- [ ] Hyprland
- [ ] MangoWC (dwl)
- [ ] Sway
- [ ] Other (specify below)

### Proposed Solution

_No response_

### Alternatives/Existing Solutions

_No response_

Start by locating the audio devices panel and the existing Niri keybind handling. Review how similar panels are opened and how compositor-specific shortcuts are configured. Done means a Niri keybind opens the audio devices panel so its controls can be accessed without navigating to it manually.

Written by the indexing model from the issue text.