Show volume/brightness OSD over fullscreen apps when using Dank Island

### Feature Description

When Dank Island is enabled, volume and brightness feedback is integrated into the Island. However, when an application is fullscreen and the Island is not using the overlay layer, changing the volume provides no visible feedback.

It would be useful if volume and brightness changes could temporarily appear above fullscreen applications, without requiring the entire Island to remain permanently on the overlay layer.

### Use Case

While gaming or watching fullscreen video, I often change the system volume with keyboard shortcuts. Currently I cannot see the current volume level unless I leave fullscreen or configure the whole Island to stay above fullscreen windows.

### Compositor(s)

Niri

### If Other, please specify

_No response_

### Proposed Solution

Allow the volume/brightness OSD portion of Dank Island to temporarily use the Wayland overlay layer when triggered, then disappear normally. Alternatively, provide a fallback standalone OSD when the Island itself is hidden by a fullscreen window.

### Alternatives/Existing Solutions

_No response_

Start by tracing the volume/brightness OSD and Dank Island's fullscreen and overlay-layer handling for Niri. Compare how the Island is hidden by fullscreen applications with how triggered notifications are displayed. Done means volume and brightness feedback appears above fullscreen apps only while active, without keeping the entire Island permanently on the overlay layer.

Written by the indexing model from the issue text.