Improved error information for the "auto" color theme

### Feature Description

For the "auto" color theme, it appears to require a wallpaper to be set explicitly. Using the default (settings menu says `No wallpaper selected`, even if a wallpaper is visible) does not work and causes everything theme-related to silently fail (light/dark mode switching included).
The improvement would be to either use the default wallpaper for color selection or show an error/warning while the auto theme is selected without a wallpaper being set.

### Use Case

As provided in the description, the auto theme silently failing if no wallpaper is explicitly set is an easy point of failure for users. This change would either remove the edge case completely or at least stop failing silently, so the user can figure out what to do.

### Compositor(s)

All compositors

### If Other, please specify

Tested on Niri, though likely applies to all compositors

### Proposed Solution

Two options:
- The "auto" color theme falls back to the default wallpaper provided if none has been set by the user
- Show an error message if the "auto" color theme is chosen and no wallpaper is explicitly set
The error message should explicitly mention either switching to the manual color theme or setting a wallpaper

### Alternatives/Existing Solutions

_No response_