Seed waypaper config into the home directory

waypaper reads only ~/.config/waypaper/config.ini and has no system-wide
fallback, so it wrote its own default with an empty wallpaper entry and
nothing was ever set. systemd.user.tmpfiles 'C' copies the file once
without overwriting later user edits.

modules/desktop.nix

@@ -75,6 +75,8 @@ in
     "xdg/gtk-4.0/settings.ini".source = "${xdg}/gtk-4.0/settings.ini";
     "xdg/pipewire/pipewire.conf.d/99-input-denoising.conf".source =
       "${xdg}/pipewire/pipewire.conf.d/99-input-denoising.conf";
+    # Kept for layout parity with the Arch package; waypaper itself only reads
+    # the copy in the home directory, seeded below.
     "xdg/waypaper/config.ini".source = waypaperConfig;
 
     # stasis reads /etc/stasis/ as its system-wide fallback, never /etc/xdg/.
@@ -239,4 +241,11 @@ in
 
   # Polkit agent for privilege prompts inside the session.
   security.polkit.enable = true;
+
+  # Seed configs for programs that read only from the home directory and have
+  # no system-wide fallback. 'C' copies once and never overwrites, so user
+  # edits survive. This is what post-install.sh does imperatively on Arch.
+  systemd.user.tmpfiles.rules = [
+    "C %h/.config/waypaper/config.ini 0644 - - - ${waypaperConfig}"
+  ];
 }