feat(iso): Point end4-pC scripts at fork and set its default wallpaper

The fork's colour/wallpaper/kvantum scripts still hard-code
QUICKSHELL_CONFIG_NAME="ii", so they read and write
~/.config/quickshell/ii instead of end4-pC - rewrite them to end4-pC
at build time (verified against the current fork HEAD: 5 scripts match
the anchored pattern, re-runs are no-ops).

Also replace the fork's assets/images/default_wallpaper.png - fed to
switchwall.sh by FirstRunExperience.qml on first login - with the
wallpaper now shipped in skel's ~/Pictures/Wallpapers, so the first
login lands on the japan-street wallpaper instead of the fork's
default.

airootfs/root/customize_airootfs.sh

@@ -134,6 +134,29 @@ if [[ ! -d "${SKEL}/.config/quickshell/end4-pC" ]]; then
   git clone --depth=1 https://github.com/pctrade/end4-pC.git \
     "${SKEL}/.config/quickshell/end4-pC"
 fi
+# The fork's colour/wallpaper/kvantum scripts still hard-code
+# QUICKSHELL_CONFIG_NAME="ii", so they'd read and write ~/.config/quickshell/ii
+# instead of end4-pC. Point them at the fork. Re-running is a no-op.
+mapfile -t PC_II_SCRIPTS < <(grep -rl --exclude-dir=.git \
+  '^QUICKSHELL_CONFIG_NAME="ii"$' "${SKEL}/.config/quickshell/end4-pC" || true)
+if (( ${#PC_II_SCRIPTS[@]} )); then
+  echo "[customize] Pointing ${#PC_II_SCRIPTS[@]} end4-pC scripts at end4-pC..."
+  sed -i 's/^QUICKSHELL_CONFIG_NAME="ii"$/QUICKSHELL_CONFIG_NAME="end4-pC"/' \
+    "${PC_II_SCRIPTS[@]}"
+fi
+# Default wallpaper: end4-pC's first run (services/FirstRunExperience.qml)
+# feeds assets/images/default_wallpaper.png to switchwall.sh, so replace that
+# image with the one shipped in skel's ~/Pictures/Wallpapers.
+DEFAULT_WALL="${SKEL}/Pictures/Wallpapers/Dynamic-Wallpapers/Dark/japan-street-Dark.png"
+PC_DEFAULT_WALL="${SKEL}/.config/quickshell/end4-pC/assets/images/default_wallpaper.png"
+if [[ ! -f "${DEFAULT_WALL}" ]]; then
+  echo "[customize] Default wallpaper ${DEFAULT_WALL} is missing" >&2
+  exit 1
+fi
+if ! cmp -s "${DEFAULT_WALL}" "${PC_DEFAULT_WALL}"; then
+  echo "[customize] Setting end4-pC default wallpaper to ${DEFAULT_WALL##*/}..."
+  install -Dm644 "${DEFAULT_WALL}" "${PC_DEFAULT_WALL}"
+fi
 
 # ── Default quickshell config: ii -> end4-pC ─────────────────────────────────
 # alvin-install copies this staged repo onto the target and runs `./setup