desktop_common: make Qt apps follow the GTK theme

Deploy /etc/profile.d/10-qt-theme.sh setting QT_QPA_PLATFORMTHEME=gtk3
system-wide. The gtk3 Qt platform theme plugin (already shipped with
qt5/qt6 base on Void) makes both Qt5 and Qt6 widgets read theme name,
font, icon theme and color scheme from ~/.config/gtk-3.0/settings.ini,
so Qt apps now match the Yaru-blue-dark setup on both i3 and sway.

Also drop the stale QT_QPA_PLATFORMTHEME=qt6ct override in nymph's
session-env (qt6ct is not installed and the override would shadow the
global gtk3 setting).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

ansible/roles/profile_desktop_common/tasks/main.yml

@@ -434,6 +434,18 @@
     - "{{ user_home }}/Maildir/iCloudAccount"
     - "{{ user_home }}/Maildir/ProtonMailAccount"
 
+- name: Make Qt apps follow the GTK theme
+  tags: [theme]
+  ansible.builtin.copy:
+    dest: /etc/profile.d/10-qt-theme.sh
+    content: |
+      # Use the gtk3 Qt platform theme so Qt5/Qt6 widgets, file dialogs,
+      # icon theme and color scheme follow ~/.config/gtk-3.0/settings.ini.
+      export QT_QPA_PLATFORMTHEME=gtk3
+    owner: root
+    group: root
+    mode: "0644"
+
 - name: Ensure flathub remote is configured
   tags: [packages]
   community.general.flatpak_remote: