feat(wallpapers-bing): restore preferred-region picker

[perlowja]

Adds a Bing Preferred Region SelectionRow to the wallpapers-bing plugin settings.

The picker obtains available markets from the helper, reads the current preference through the helper, and delegates preference updates to the helper without accessing distro-specific configuration files.

Verified by building wallpapers-bing/libwallpapers-bing.so with Meson and Ninja in a Debian trixie container against the current libsingularity fallback.

AI assistance: disclosed

[perlowja]

@mirkobrombin bumping this one too - restores the preferred-region picker for the Bing wallpaper source.

[mirkobrombin]

Singularity must stay distro-agnostic: the plugin shouldn't write another distro's config (~/.config/ncz-wallpaper/bing-markets) or hardcode the market list. Get markets from the helper's markets command and persist the choice through the helper. Also: bing_markets_updating is never set, use string.ascii_down() instead of ascii_lower, and drop the cix-installer/zoder references from comments.

[perlowja]

@mirkobrombin fixed in c7c10c7 (squashed to one commit): markets come from the helper, bing_markets_updating is gone, ascii_down() is used, and the commit message no longer names any downstream project.

[perlowja]

@mirkobrombin rebased onto main after today's merges. The only conflict was the helper lookup from 869d2af; the provider now uses singularity-wallpaper-bing like the other plugins. The change itself is the same as before. wallpapers-bing builds on Debian forky.