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/gtk-theme-set.sh

@@ -117,3 +117,16 @@ if command -v dconf >/dev/null 2>&1; then
     dconf write /org/gnome/desktop/interface/icon-theme "'$icon_theme'"
     dconf write /org/gnome/desktop/interface/color-scheme "'$color_scheme'"
 fi
+
+# GTK2 compat: separate config format (~/.gtkrc-2.0, not settings.ini),
+# and no live theme-switching mechanism at all — GTK2 apps only read this
+# at their own startup. Only icon theme and font actually follow the
+# current theme; gnome-themes-extra's bundled GTK2 Adwaita engine has no
+# dark variant to switch to (GTK2 predates that concept), so gtk-theme-
+# name here is always the one real option, not a broken attempt at dark
+# mode.
+cat > "$HOME/.gtkrc-2.0" <<EOF
+gtk-theme-name="Adwaita"
+gtk-icon-theme-name="$icon_theme"
+gtk-font-name="Noto Sans 10"
+EOF