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/theme.nix

@@ -13,6 +13,16 @@
     adwaita-icon-theme
     gruvbox-plus-icons
     dconf
+    # Qt's side of the same live-theming pipeline as GTK above — see
+    # qt-theme-set (modules/scripts/qt-theme-set.sh). Without this
+    # installed, QT_QPA_PLATFORMTHEME=qt6ct (hyprland.conf) pointed at a
+    # platform theme plugin that didn't exist, so every Qt app (Gwenview,
+    # Ark, Kate, ...) silently fell back to its own default style/palette
+    # instead of anything resembling the rest of the desktop.
+    qt6Packages.qt6ct
+    kdePackages.qqc2-desktop-style # QQC2 style needed for QT_QUICK_CONTROLS_STYLE=
+                                     # org.kde.desktop (hyprland.conf) to actually
+                                     # theme Kirigami-based apps' chrome
   ];
 
   # No environment.sessionVariables.GTK_THEME here on purpose: GTK_THEME
@@ -25,11 +35,16 @@
   systemd.tmpfiles.rules = [
     "d /home/nexoniarz/.config/gtk-3.0 0755 nexoniarz users -"
     "d /home/nexoniarz/.config/gtk-4.0 0755 nexoniarz users -"
+    "d /home/nexoniarz/.config/qt6ct 0755 nexoniarz users -"
+    "d /home/nexoniarz/.config/qt6ct/colors 0755 nexoniarz users -"
     # `f`, not `L+`: created empty only if missing, never overwritten on
-    # rebuild — gtk-theme-set owns the actual content from here on.
+    # rebuild — gtk-theme-set/qt-theme-set own the actual content from
+    # here on.
     "f /home/nexoniarz/.config/gtk-3.0/gtk.css 0644 nexoniarz users - -"
     "f /home/nexoniarz/.config/gtk-4.0/gtk.css 0644 nexoniarz users - -"
     "f /home/nexoniarz/.config/gtk-3.0/settings.ini 0644 nexoniarz users - -"
     "f /home/nexoniarz/.config/gtk-4.0/settings.ini 0644 nexoniarz users - -"
+    "f /home/nexoniarz/.gtkrc-2.0 0644 nexoniarz users - -"
+    "f /home/nexoniarz/.config/qt6ct/qt6ct.conf 0644 nexoniarz users - -"
   ];
 }