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.

AGENTS.md

@@ -116,7 +116,7 @@ commit; CI remains the enforcer. Bypass it with `git commit --no-verify`.
 - The **image** ships its own `[local-aur]` at `file:///opt/localrepo` (see `.live-pacman.conf`). It holds `localrepo/` (copied by CI/`update.sh`) plus everything `install-deps` built. pacstrap runs without `-P`, so the target keeps the stock `/etc/pacman.conf`; `alvin-install` strips `[local-aur]` from it anyway as a safety net, because `/opt/localrepo` exists only on the medium.
 - `alvin-install` runs end-4 with `--skip-alldeps` — dependencies must come from the ISO. It feeds `yes i` to end-4's failure prompt: inside the installer chroot the medium's `/run` is bind-mounted in, so end-4's `systemctl --machine=…@.host --user enable … --now` lines can never work (and `--now` would start services on the live medium). `install_end4_dots` re-applies the enable-only part (bluetooth, ydotool, i2c module) outside the chroot and reports every step end-4 had to skip. If the end-4 step reports "incomplete", check that `hyprctl`/`quickshell`/the venv were resolvable (harvest) before re-running anything.
 - `/etc/skel/.cache/uv` + `/etc/skel/.local/share/uv/python` are pre-warmed by the build (~460 MB: CPython 3.12 and the pinned wheel cache) so end-4's `uv venv`/`uv pip install` at install time needs no network.
-- **end4-pC** (`pctrade/end4-pC`) is the default quickshell config: cloned into `/etc/skel/.config/quickshell/end4-pC` at build time, selected by patching `hl.env("qsConfig", "ii")` → `"end4-pC"` in the **staged** `/opt/dots-hyprland/dots/.config/hypr/hyprland/variables.lua` (there is no `hypr/hyprland/` in this repo — `./setup install` copies it to the installed system). `alvin-install` re-copies end4-pC into the user's home *after* `./setup install`, because end-4's installer `rsync --delete`s all of `~/.config/quickshell`.
+- **end4-pC** (`pctrade/end4-pC`) is the default quickshell config: cloned into `/etc/skel/.config/quickshell/end4-pC` at build time, selected by patching `hl.env("qsConfig", "ii")` → `"end4-pC"` in the **staged** `/opt/dots-hyprland/dots/.config/hypr/hyprland/variables.lua` (there is no `hypr/hyprland/` in this repo — `./setup install` copies it to the installed system). `alvin-install` re-copies end4-pC into the user's home *after* `./setup install`, because end-4's installer `rsync --delete`s all of `~/.config/quickshell`. The build also rewrites the fork's hard-coded `QUICKSHELL_CONFIG_NAME="ii"` in its colour/wallpaper/kvantum scripts to `end4-pC` (otherwise they act on `~/.config/quickshell/ii`), and overwrites its `assets/images/default_wallpaper.png` (applied by `FirstRunExperience.qml` on first login) with `airootfs/etc/skel/Pictures/Wallpapers/Dynamic-Wallpapers/Dark/japan-street-Dark.png`.
 - `airootfs/etc/skel/.config/illogical-impulse/presets/` carries the user's presets (`Laptop.json`, `Personal-Performance.json`, `Personal-Performance.zip`). They embed `/home/alvin/...` paths, so wallpapers/screenshots need fixing up after install.
 - End-4 metapackages are built from `localrepo/end4-pkgbuilds/` (quickshell-git, bibata-modern-classic-bin, microtex-git).
 - **quickshell pin / Qt 6.12**: end-4 pins quickshell to `41651d7` (Sep 25), which predates upstream fix `5d5d498` (Oct 3, *"use MOC includes instead of opaque pointer definitions — … Qt 6.12 compatibility"*). Arch shipped qt6 6.12.0 on Oct 8 and the old pin died with `Meta Types must be fully defined` in the moc build of quickshell-hyprland-ipc (failed CI run 137). `customize_airootfs.sh` therefore rewrites end-4's `_commit` to `11ca60be` after cloning — but **only** while it still matches `^_commit='41651d7`, so once end-4 bumps their own pin the guard goes inert and theirs wins. `localrepo/check-quickshell-pin.sh` (weekly `pin-check.yaml` job) fails CI if either pin goes stale or loses the fix; it warns instead of failing when end-4's known-stale pin is covered by the override.