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/init.sh

@@ -2,8 +2,8 @@
 set -euo pipefail
 
 if [[ "${EUID:-$(id -u)}" -ne 0 ]]; then
-  echo "[installation/install] Warning: Installation is just possible via root."
-  exit 0
+  echo "[installation/install] ERROR: Installation requires root. Re-run with sudo." >&2
+  exit 1
 fi
 
 echo "[installation] Running as root (EUID=0)."