Switch greeter from tuigreet to regreet

- Replace greetd-tuigreet with greetd-regreet + cage in packages.txt
- Add greetd/config.toml and greetd/regreet.toml (Materia-dark theme)
- Update install.sh to deploy greetd configs to /etc/greetd/
- Fix powermenu.sh awk parsing (use $NF instead of $2 for icon glyphs)

greetd/regreet.toml

@@ -0,0 +1,13 @@
+[GTK]
+application_prefer_dark_theme = true
+cursor_theme_name = "Adwaita"
+font_name = "Sans 10"
+icon_theme_name = "Papirus-Dark"
+theme_name = "Materia-dark"
+
+[commands]
+reboot = ["systemctl", "reboot"]
+poweroff = ["systemctl", "poweroff"]
+
+[appearance]
+greeting_msg = "Welcome back!"