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

dotfiles/quickshell/Theme.qml

@@ -104,6 +104,11 @@ Item {
             theme.dark ? "1" : "0"];
         gtkSyncProc.running = true;
 
+        qtSyncProc.command = ["qt-theme-set",
+            theme.accent, theme.bg, theme.bgAlt, theme.fg, theme.fgDim, theme.border,
+            theme.dark ? "1" : "0"];
+        qtSyncProc.running = true;
+
         kittySyncProc.command = ["kitty-theme-set",
             theme.accent, theme.bg, theme.bgAlt, theme.fg, theme.fgDim, theme.border];
         kittySyncProc.running = true;
@@ -117,6 +122,7 @@ Item {
     Process { id: saveProc }
     Process { id: hyprSyncProc }
     Process { id: gtkSyncProc }
+    Process { id: qtSyncProc }
     Process { id: kittySyncProc }
     Process { id: rofiSyncProc }
     Process {