default-flatpaks: notify off — module's notify path breaks multi-user

The v1 module's post-install notification runs `sudo -u` with every
human user newline-joined into a single name ("kid\nparent"), which
fails and kills the service mid-install on any box with two accounts.
Hit on first hardware boot; apps after the first never installed.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

recipes/bc250-console-gnome.yml

@@ -28,7 +28,11 @@ modules:
       - dconf update # compile the local.d/gdm.d console defaults into the dconf dbs
 
   - type: default-flatpaks@v1
-    notify: true
+    # notify: true is broken on multi-user systems (kid + parent): the
+    # module's per-app notification does `sudo -u` with all users mashed
+    # into one newline-joined string -> "unknown user" -> service fails
+    # mid-install. Seen on first hardware boot 2026-07-13.
+    notify: false
     system:
       install:
         - net.davidotek.pupgui2 # ProtonUp-Qt: Proton-GE manager