Add Qt/GTK2 theming, default apps, LibreOffice; fix display.conf race

- Add qt-theme-set: a working Qt theming pipeline via qt6ct (the
  QT_QPA_PLATFORMTHEME=qt6ct env var was pointing at a platform theme
  plugin that wasn't even installed, so every Qt app fell back to its
  own default style). Also regenerates kdeglobals' KColorScheme groups
  and sets QT_QUICK_CONTROLS_STYLE=org.kde.desktop for Kirigami-based
  KDE apps, whose QML-drawn chrome ignores QPalette/qt6ct entirely.
  Gwenview's dialogs/content now follow the theme; its toolbar/statusbar
  chrome is a known remaining gap tied to Kirigami's platform-theme
  detection outside a full Plasma session
- Add GTK2 compat (~/.gtkrc-2.0: icon theme + font follow the theme;
  no dark variant exists for GTK2's Adwaita engine)
- Add modules/default-apps.nix: Gwenview/Ark as system-default image
  viewer/archive manager via /etc/xdg/mimeapps.list, LibreOffice
  (Qt6 build), and a working Thunar "Open Terminal Here" (it shipped
  calling exo-open, which isn't installed and never resolved)
- Fix display-set.sh: mktemp defaulted to /tmp, a different filesystem
  than ~/.config/hypr here, making the mv into display.conf a
  non-atomic copy+unlink. Hyprland's file watcher could catch it
  mid-replace and throw "source= globbing error: found no match" on
  any monitor mode/orientation change. Stress-tested with 60 concurrent
  write+reload cycles after the fix with zero errors

modules/scripts.nix

@@ -12,6 +12,7 @@ let
   displaySet = pkgs.writeShellScriptBin "display-set" (builtins.readFile ./scripts/display-set.sh);
   hyprThemeSet = pkgs.writeShellScriptBin "hypr-theme-set" (builtins.readFile ./scripts/hypr-theme-set.sh);
   gtkThemeSet = pkgs.writeShellScriptBin "gtk-theme-set" (builtins.readFile ./scripts/gtk-theme-set.sh);
+  qtThemeSet = pkgs.writeShellScriptBin "qt-theme-set" (builtins.readFile ./scripts/qt-theme-set.sh);
   kittyThemeSet = pkgs.writeShellScriptBin "kitty-theme-set" (builtins.readFile ./scripts/kitty-theme-set.sh);
   rofiThemeSet = pkgs.writeShellScriptBin "rofi-theme-set" (builtins.readFile ./scripts/rofi-theme-set.sh);
   listCursorThemes = pkgs.writeShellScriptBin "list-cursor-themes" (builtins.readFile ./scripts/list-cursor-themes.sh);
@@ -36,6 +37,7 @@ in
     displaySet
     hyprThemeSet
     gtkThemeSet
+    qtThemeSet
     kittyThemeSet
     rofiThemeSet
     listCursorThemes