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/theme.nix

@@ -44,11 +44,24 @@ let
       blur = true;
       shadow = true;
     };
+    # Icon theme name; "" means "pick by mode" (resolved below). A preset
+    # or the state file can name any theme in the shipped icon package.
+    icons = "";
   };
+
+  parsed = lib.recursiveUpdate defaults themeState;
+
+  # Resolve the icon theme once and expose it on nomarchy.theme so both
+  # the GTK side (stylix.nix) and rofi (rofi.nix) agree. The shipped
+  # package (papirus-icon-theme) carries the Dark/Light pair.
+  iconTheme =
+    if parsed.icons != "" then parsed.icons
+    else if parsed.mode == "light" then "Papirus-Light"
+    else "Papirus-Dark";
 in
 {
   config = {
-    nomarchy.theme = lib.recursiveUpdate defaults themeState;
+    nomarchy.theme = parsed // { inherit iconTheme; };
 
     nomarchy.lib = {
       # "#7aa2f7" -> "rgb(7aa2f7)"          (Hyprland color syntax)