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/stylix.nix

@@ -55,6 +55,17 @@ in
         size = lib.mkDefault 24;
       };
 
+      # GTK/file-manager/rofi icon theme, resolved from the JSON in
+      # theme.nix (per-theme `icons`, else Papirus-Dark/Light by mode).
+      # Stylix sets gtk.iconTheme from this; both dark/light point at the
+      # already-mode-resolved name.
+      icons = {
+        enable = true;
+        package = lib.mkDefault pkgs.papirus-icon-theme;
+        dark = t.iconTheme;
+        light = t.iconTheme;
+      };
+
       fonts = {
         monospace = {
           name = t.fonts.mono;