fix(install): use runuser and fail non-root with exit 1

`su -` runs through pam_systemd on Manjaro/Arch, creating a new login
session that conflicts with the outer sudo session and detaches the
install from the controlling terminal — making `sudo make install`
appear to end while it keeps running in the background. Replace `su`
calls with `runuser`, which is designed for root-invoked scripts and
skips PAM session management.

Also flips init.sh's non-root branch from `exit 0` (silent success) to
`exit 1` with a clear stderr message, so `make install` correctly fails
when invoked without root.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

scripts/installation/arch/package.sh

@@ -47,7 +47,7 @@ echo "[arch/package] Using 'aur_builder' user for makepkg..."
 chown -R aur_builder:aur_builder "${BUILD_ROOT}"
 
 echo "[arch/package] Running makepkg in: ${PKG_BUILD_DIR}"
-su aur_builder -c "cd '${PKG_BUILD_DIR}' && rm -f package-manager-*.pkg.tar.* && makepkg --noconfirm --clean --nodeps"
+runuser -u aur_builder -- bash -c "cd '${PKG_BUILD_DIR}' && rm -f package-manager-*.pkg.tar.* && makepkg --noconfirm --clean --nodeps"
 
 echo "[arch/package] Installing generated Arch package..."
 pkg_path="$(find "${PKG_BUILD_DIR}" -maxdepth 1 -type f -name 'package-manager-*.pkg.tar.*' | head -n1)"