feat: icon themes — Papirus, resolved from the JSON, for GTK + rofi

Ships papirus-icon-theme and resolves an icon-theme name once in
theme.nix → nomarchy.theme.iconTheme: the JSON's optional `icons` field,
else Papirus-Dark/Light picked by `mode`. That single value feeds:
- Stylix `stylix.icons` → gtk.iconTheme (Thunar and GTK apps)
- rofi `show-icons` + `icon-theme` (the launcher now shows app icons)

A preset or theme-state.json can set `icons` to any theme in the icon
package to override per theme.

Verified: flake check green; iconTheme resolves Papirus-Dark for
tokyo-night and Papirus-Light for summer-day; gtk.enable true with
gtk.iconTheme.name set; papirus in home.packages and the live ISO
closure; rofi config.rasi carries icon-theme + show-icons:true. The
rendered icons need a real session to see.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

modules/home/rofi.nix

@@ -131,7 +131,8 @@ in
 
       extraConfig = {
         modi = "drun,run";
-        show-icons = false; # no icon theme shipped yet — clean text list
+        show-icons = true;          # app icons via the theme's icon set
+        icon-theme = t.iconTheme;   # resolved in theme.nix (Papirus-*/per-theme)
         drun-display-format = "{name}";
         display-drun = "apps";
         display-run = "run";