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/config.toml

@@ -0,0 +1,6 @@
+[terminal]
+vt = 1
+
+[default_session]
+command = "cage -s -- regreet"
+user = "greeter"