CmdPal: Dock: Add optional Dock separator for a more taskbar-like appearance

### Description of the new feature / enhancement

Add an optional subtle separator between CmdPal Dock and the content area.

The goal is to make CmdPal Dock feel closer to the Windows taskbar. The Windows taskbar has a subtle boundary between the taskbar and app content, which helps separate the persistent shell surface from the active content area. CmdPal Dock could benefit from a similar optional separator, especially when the Dock and content area visually blend together.

### Scenario when this would be used?

When enabled, the separator should appear only on the edge where the Dock touches the content area:

- Bottom Dock: separator on the top edge.
- Top Dock: separator on the bottom edge.
- Left Dock: separator on the right edge.
- Right Dock: separator on the left edge.

This would improve the visual hierarchy between the Dock and the content area. It would make the Dock feel more like a persistent system-level surface, while keeping the UI lightweight and unobtrusive.

### Supporting information

Add a toggle such as:

`Show Dock separator`

The separator should be subtle, about 1 effective pixel, and should not look visually heavy. It should not change Dock size, spacing, or layout behavior.

When the option is disabled, CmdPal Dock should keep the current appearance.

Start by locating CmdPal Dock's settings entry point and the code that renders the Dock for each edge. Add an optional “Show Dock separator” setting and verify that enabling it places a subtle one-effective-pixel separator only on the edge touching the content area, without changing size, spacing, or layout; disabled behavior should remain unchanged.

Written by the indexing model from the issue text.