Per bar frame size configuration

### Feature Description

Hi !

When having multiple frame bars, it would be great if we could control the frame size

Right now, adjusting the frame width changes it to all bars, despite the helper message to add a bar stating "Each bas has its own styling".

### Use Case

I use my frame main bar on the left side. I wanted to have a top bar also to add my media widget in the center.
However having a shared size between horizontal & vertical bar does not render beautifuly

### Compositor(s)

Niri

### If Other, please specify

_No response_

### Proposed Solution

Adding a bar selector to choose on which bar frame styling options will apply, like in Appearance tab

### Alternatives/Existing Solutions

_No response_

Start by locating the frame styling controls and the existing Appearance tab bar selector, then trace how frame width is shared across bars. Done means horizontal and vertical bars can retain independent frame sizes without changing one another.

Written by the indexing model from the issue text.