hyprpaper: fix wallpaper IPC script (use writeShellScript)

The previous inline shell command had quoting/PATH issues. Replace with
a proper pkgs.writeShellScript that:
- Uses the full hyprctl store path (no PATH dependency)
- Reads the wallpaper from hyprpaper.conf at runtime ($HOME/.config)
- Avoids the Nix string-context conflict from embedding a store path

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

settings/hyprland.nix

@@ -206,6 +206,13 @@ in
 
         extraConfig =
           let
+            # hyprpaper 0.8.4 bug: wallpaper= in config is silently ignored.
+            # preload= works; set the wallpaper via IPC after startup.
+            setWallpaper = pkgs.writeShellScript "set-hyprpaper-wallpaper" ''
+              sleep 2
+              wallpaper=$(grep '^preload=' "$HOME/.config/hypr/hyprpaper.conf" | head -1 | cut -d= -f2-)
+              ${hyprland-pkgs.hyprland}/bin/hyprctl hyprpaper wallpaper "${if isGaming then "DP-2" else "eDP-1"},$wallpaper"
+            '';
             powerMenu = pkgs.writeShellScript "power-menu" ''
               choice=$(printf '%s\n' \
                 $'\uf023  Lock' \
@@ -268,11 +275,7 @@ in
               hl.exec_cmd("hyprctl setcursor Bibata-Modern-Ice 24")
               hl.exec_cmd("swayosd-server")
               ${lib.optionalString isMacbook ''hl.exec_cmd("hypridle")''}
-              -- hyprpaper 0.8.4 bug: wallpaper= in config is ignored on startup.
+              hl.exec_cmd("${setWallpaper}")
-              -- preload= works so the image is ready; use IPC to assign it.
-              -- Read the path from hyprpaper.conf at runtime to avoid Nix
-              -- string-context conflicts with the powerMenu derivation above.
-              hl.exec_cmd("sleep 2 && hyprctl hyprpaper wallpaper '${if isGaming then "DP-2" else "eDP-1"},'$(grep '^preload=' \"$XDG_CONFIG_HOME/hypr/hyprpaper.conf\" | head -1 | cut -d= -f2-)")
             end)
 
             -- Animation curve and definitions