fix: make 20_storage.sh self-contained like every other module

It called log_info/log_warn/error_exit/list_install_disks without
sourcing anything - leftover from before modules were run as
standalone subprocesses via run_module() rather than sourced into
installer.sh. Never noticed because the pipeline never reached this
module in a live run until now. Behavior unchanged: still just lists
disks and warns that partitioning is disabled (Phase 1).

scripts/modules/20_storage.sh

@@ -1,14 +1,37 @@
 #!/usr/bin/env bash
+set -Eeuo pipefail
 
-log_info "Datenträger werden erkannt..."
+SCRIPT_NAME="$(basename "${BASH_SOURCE[0]}")"
+readonly SCRIPT_NAME
 
-DISKS="$(list_install_disks || true)"
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+readonly SCRIPT_DIR
 
-if [[ -z "$DISKS" ]]; then
-  error_exit "Keine geeigneten Datenträger erkannt."
-fi
+# shellcheck source=../lib/checks.sh
+source "${SCRIPT_DIR}/../lib/checks.sh"
 
-echo "$DISKS"
+log() {
+    printf '[%s] %s\n' "${SCRIPT_NAME}" "$*"
+}
 
-log_warn "Phase 1: Datenträger werden nur angezeigt, nicht verändert."
-log_warn "Partitionierung ist noch deaktiviert."
+fatal() {
+    printf '[%s] FEHLER: %s\n' "${SCRIPT_NAME}" "$*" >&2
+    exit 1
+}
+
+main() {
+    log "Datenträger werden erkannt..."
+
+    local disks
+    disks="$(list_install_disks || true)"
+
+    [[ -n "${disks}" ]] ||
+        fatal "Keine geeigneten Datenträger erkannt."
+
+    printf '%s\n' "${disks}"
+
+    log "Phase 1: Datenträger werden nur angezeigt, nicht verändert."
+    log "Partitionierung ist noch deaktiviert."
+}
+
+main "$@"