Remove remaining OS-specific hardcodes from core scripts

OS version reads now go through platform_get_os_version() and
platform_os_version_probe_cmd() instead of grepping /etc/unraid-version directly.

STATE_DIR fallbacks to /boot/config removed — STATE_DIR is always set by
load_config.sh and the fallback encoded a platform-specific path.

Setup DB path references use platform_setup_db_path() instead of the
VARAVERK_SETUP_FILE/-/boot/config compound fallback.

DOCKER_APPDATA_BASE default removed from arr_sync.sh — the adapter sets it.

Partnership/onboard_cancel.sh

@@ -54,7 +54,7 @@ OWNER_ID="${PARTNERSHIP_OWNER_HOST:-HOST1}"
 MIRROR_ID=$( [[ "$OWNER_ID" == "HOST1" ]] && echo "HOST2" || echo "HOST1" )
 MIRROR="${!MIRROR_ID}"
 SSH_KEY_PUB="${SSH_KEY}.pub"
-STATE_FILE="${VARAVERK_SETUP_FILE:-${STATE_DIR:-/boot/config}/varaverk_setup.db}"
+STATE_FILE="$(platform_setup_db_path)"
 AUTH_KEYS="/root/.ssh/authorized_keys"
 MIRROR_SHORT="${MIRROR%%.*}"