Add Pi theme based on system theme

Based entirely on the work of @robzolkos in #5711. That attempt was
adapted to better fit the standard templating system while also adding
color mixing for use in other themes.

install/config/pi.sh

@@ -1,2 +1 @@
-mkdir -p ~/.pi/agent/extensions
-cp "$OMARCHY_PATH/default/pi/agent/extensions/omarchy-system-theme.ts" ~/.pi/agent/extensions/
+omarchy-theme-set-pi --activate