Add font-name and xft-dpi controls to the Login Window GUI (match desktop appearance)

[sporteka2]

Problem

The login screen (slick-greeter) runs as the lightdm system user in a separate session. It does not inherit the logged-in user's font and DPI settings (org.gnome.desktop.interface font-name, text-scaling-factor, and Xft.dpi). It only reads /etc/lightdm/slick-greeter.conf, so by default the greeter uses its own font and auto-DPI — which looks different from the desktop the user just configured.

Existing capability, not exposed in the GUI

slick-greeter.conf already supports the keys font-name and xft-dpi (plus xft-antialias, xft-hintstyle, xft-rgba). However, lightdm-settings only exposes a "HiDPI support" toggle (enable-hidpi: auto/on/off) and provides no explicit font or DPI controls (see the "Settings" page in lightdm-settings).

Request

Add a Fonts / Appearance section to the Login Window tool with:

• GTK font name (font-name)
• DPI (xft-dpi)
• Ideally a "Use same as current session" checkbox that, on save, reads the current user's org.gnome.desktop.interface font-name and computed DPI (from text-scaling-factor / Xft.dpi) and writes them into slick-greeter.conf.

Why

• Consistent look between the login screen and the desktop.
• Important for HiDPI and accessibility users (e.g. a 1.6× text scale / ~307 DPI desktop currently falls back to a tiny greeter font).

Environment (reporter)

• Linux Mint, Cinnamon (X-Cinnamon)
• lightdm-settings 2.1.1
• slick-greeter 2.2.6