Refactor first-boot wallpaper setup to use pcmanfm configuration instead of one-shot scripts. Update documentation to reflect changes in wallpaper handling and remove obsolete one-shot references. Enhance logging for desktop wallpaper setup during first boot.

chromium-setup/emmc-provisioning/cloud-init/files-from-guard/README.md

@@ -15,7 +15,8 @@ first-boot.sh downloads from **`.../files/first-boot/`** (e.g. `http://10.130.60
 | **99-default-session.conf** | LightDM default session rpd-labwc (from `config-files/`). |
 | **maliit-keyboard.desktop** | Maliit on-screen keyboard autostart (from `config-files/`). |
 | **set-rotation-once.sh** + **.desktop** | One-shot: wlr-randr rotation (Left) at first login. |
-| **set-wallpaper-once.sh** + **.desktop** | One-shot: swaybg wallpaper + labwc autostart at first login. |
+
+Desktop wallpaper is set once during first-boot (labwc autostart created by first-boot.sh); no set-wallpaper-once needed.
 
 ---
 
@@ -25,4 +26,4 @@ first-boot.sh downloads from **`.../files/first-boot/`** (e.g. `http://10.130.60
 - **plymouth-custom/custom.plymouth** — Plymouth theme definition; host as `custom.plymouth`.
 - **plymouth-custom/custom.script** — Plymouth script that draws splash.png; host as `custom.script`.
 - **lightdm/RPiSystem_dark.png** — Unused; only `splash.png` is used now.
-- **set-rotation-once.sh** and **set-wallpaper-once.sh** live in `cloud-init/`; copy them into `portal-files/first-boot/` on the file server.
+- **set-rotation-once.sh** lives in `cloud-init/`; copy it (and its .desktop) into `portal-files/first-boot/` on the file server.