[Feature Request]: Add transparency setting to lines .

### Is there an existing issue for this?

- [x] I have searched the existing issues and checked the recent builds/commits

### What would your feature do ?

Add transparency setting to lines.
Problem: When the workflow becomes complicated, the lines spread like a spider web.

### Proposed workflow

Solution: If you can set the transparency for the non-selected lines, such as sliding the slider to 10%~50%, this will make the lines less prominent visually. The entire ComfyUI interface looks simpler (visually). Much better than just "hiding"!

When you operate, the transparency of the selected lines is automatically 100% (highlighted), which does not affect the editing experience.

Example: I simulated the effect using Photoshop, with the line transparency at 10%.

### Additional information

In summary, by increasing the transparency of the lines, the user experience can be improved visually.

Add the setting here:

┆Issue is synchronized with this [Notion page](https://www.notion.so/Issue-3819-Feature-Request-Add-transparency-setting-to-lines-1ed6d73d3650816c8089caf69e195f3d) by [Unito](https://www.unito.io)

The issue does not name files or tests. Start by locating the frontend code that renders workflow lines and the settings UI shown in the screenshots; define done as a setting that dims non-selected lines within the requested range while keeping selected lines fully visible.

Written by the indexing model from the issue text.