Add Padding Option In Dank-bar Similar To Dock

### Feature Description

This Will Allow User To Add Spacing Between Widgets With Border & Dank-bar's Height.

### Use Case

Widgets With Thick Border Will Look Proper.

### Compositor(s)

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

### Proposed Solution

Add Padding Option Similar To Dock

### Alternatives/Existing Solutions

Maybe adding Option To Maintain Certain Padding Even If Border Is Added.

Start by comparing the existing dank-bar and Dock options to understand how Dock maintains padding around bordered widgets. Define the padding behavior for the checked compositor cases and verify that the bar height and widget spacing remain correct when borders are present.

Written by the indexing model from the issue text.