draw-user-backgrounds: solid desktop colour not shown on login screen

Description

When the user sets a solid colour as desktop background (Picture: None), the login screen with draw-user-backgrounds=true does not show the chosen colour. It shows the old wallpaper (from picture-uri) instead.

Steps

1. Cinnamon Settings → Background → Picture: None, pick a colour
2. Lock screen or log out
3. Login screen shows the old wallpaper, not the solid colour

Root Cause

When the user selects a solid colour, Cinnamon only sets picture-options=none and primary-color, but does not clear or update picture-uri. It still points to the old wallpaper.

Slick-greeters draw-user-backgrounds reads the users background via LightDM → AccountsService BackgroundFile. This file is empty when there is no wallpaper. But LightDM may still fall back to the old picture-uri, so slick-greeter loads and displays the old wallpaper file instead of the chosen solid colour.

BackgroundLoader.load() supports hex colour strings natively. The fix: when picture-options=none, pass primary-color to the background loader instead of picture-uri.

Data

BackgroundFile (AccountsService) = "" (empty)
picture-uri                       = file:///...jbaer_wave.jpg (old wallpaper, not cleared)
picture-options                   = none
primary-color                     = #2626a2a26969
Login screen shows                = jbaer_wave.jpg (old wallpaper)

System

Slick-greeter 2.2.6, Cinnamon 6.4.8 (Linux Mint 22.1)

Start at the BackgroundLoader.load() path and inspect how the login background chooses between picture-uri and primary-color when picture-options is none. Reproduce the solid-colour setup described in the issue, then verify that the login screen shows the selected colour instead of the old wallpaper while wallpaper backgrounds still work.