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/display-set.sh

@@ -25,7 +25,15 @@ touch "$conf"
 
 line="monitor = $descriptor"
 if grep -q "^monitor = $mon," "$conf" 2>/dev/null; then
-    tmp=$(mktemp)
+    # mktemp in the same directory as $conf, not the default /tmp: /tmp is
+    # a different filesystem here, which makes `mv` below a non-atomic
+    # copy+unlink instead of a rename — Hyprland's file watcher (it
+    # watches every `source=`d file, including this one) could catch the
+    # destination mid-replace and its `source=` glob would find nothing,
+    # throwing "globbing error: found no match" (observed live after an
+    # orientation change, which always hits this branch since DP-1
+    # already has an existing line by then).
+    tmp=$(mktemp "$conf.XXXXXX")
     awk -v prefix="monitor = $mon," -v newline="$line" '
         index($0, prefix) == 1 { print newline; next } { print }
     ' "$conf" > "$tmp"