refactor: best_effort log for quiet steps, registered temp dirs, one exit handler

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ss5gKj7XG5JPi27XwGBGTs

lib/single-user.sh

@@ -42,7 +42,7 @@ single_wallet_enable() {
         ok "Single user mode's wallet (no password) is back; your own is kept as kdewallet.kwl$WALLET_BACKUP."
         return 0
     fi
-    tmp="$(mktemp -d)"
+    make_tmpdir tmp
     if ! fetch_valve_presets "$tmp"; then
         rm -rf "$tmp"
         warn "Couldn't get Valve's empty wallet; apps may still ask for the wallet password."