Request: Let the user pick the Matugen source color arbitrarily

### Feature Description

Currently, Matugen is only usable in way that extracts the "source color" from a corresponding wallpaper. It would be great to have a possibility to let the user pick the source color arbitrarily

### Use Case

I assume many users change wallpapers often (either manually, via the "Automatic Cycling" feature or via plugins such as "Wallpaper of the Day") but still would prefer a more stable UI theming. Matugen generates excellent themes.

### Compositor(s)

All compositors

### If Other, please specify

_No response_

### Proposed Solution

The only interesting question is how to present this to the user.

A color picker should be added somewhere. I'm just not entirely sure in which tab it should belong.

I think one nice way would be to rename "Auto" to "Based on a color" and then give the user two options: 1) Take the color from the wallpaper or 2) pick one.

Alternatively, split it into two tabs "Based on a color" and "Based on wallpaper color". (Or "From a color" / "From wallpaper color").

### Alternatives/Existing Solutions

#3020 provides more control over what color is extracted from the wallpaper. But this is still very different from ignoring the wallpaper and letting the user pick.

Review the existing Matugen settings UI and the current Auto mode that derives a source color from wallpaper. Decide how the color-picker option should be presented alongside wallpaper-derived colors, then verify that an arbitrary source color can be selected and remains independent of wallpaper changes.

Written by the indexing model from the issue text.