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

hyprland/hyprland.conf

@@ -42,6 +42,13 @@ env = XDG_SESSION_DESKTOP,Hyprland
 env = GDK_BACKEND,wayland,x11
 env = QT_QPA_PLATFORM,wayland;xcb
 env = QT_QPA_PLATFORMTHEME,qt6ct
+# Kirigami-based KDE apps (confirmed live: Gwenview's toolbar/statusbar)
+# draw their chrome in QML, not QWidget — outside a real Plasma session
+# they default to their own hardcoded light "Basic" QQC2 style instead of
+# reading qt6ct or kdeglobals at all. org.kde.desktop is the QQC2 style
+# that actually pulls from KColorScheme/kdeglobals like the rest of the
+# theming pipeline here does.
+env = QT_QUICK_CONTROLS_STYLE,org.kde.desktop
 
 # --- Input ------------------------------------------------------------
 input {