fix(tariff): reject stepped base combined with time/seasonal tiers

A stepped ("up-to") default card prices the whole stay as one total, so the V2
engine short-circuits to steppedFee and NEVER consults windowed cards — any
time/seasonal tiers would silently never fire. Found live: an active tariff had a
stepped base plus weekday-night + weekend tiers, and every 3h stay priced 600 ALL
regardless of hour/day because the tiers were dead.

- validateTariffV2 now rejects a stepped defaultCard combined with windowedCards,
  with an actionable message (switch the base to ladder/flat, or remove the tiers).
- Composer shows an inline red warning the moment base mode is stepped and tiers
  exist; publishing is blocked server-side regardless.
- ApiError now carries the server's problems[], so the publish error surfaces the
  SPECIFIC reason instead of a generic "invalid tariff structure".
- 2 new validation tests (55 pass).

Wiki: tariff, log.

Claude-Session: https://claude.ai/code/session_01Xcm6ikLgGoCxxHrxtjkk5V

apps/web/src/TariffComposer.tsx

@@ -341,8 +341,8 @@ export function TariffComposer() {
       setMsg({ kind: "ok", text: t("tariff.publishedOk") });
     } catch (e) {
       const text =
-        e instanceof ApiError && (e as ApiError & { problems?: string[] }).problems
-          ? `${e.message}: ${((e as ApiError & { problems?: string[] }).problems ?? []).join("; ")}`
+        e instanceof ApiError && e.problems?.length
+          ? `${e.message}: ${e.problems.join("; ")}`
           : (e as Error).message;
       setMsg({ kind: "err", text });
     } finally {
@@ -404,6 +404,13 @@ export function TariffComposer() {
       <details className="mt-6" open={form.tiers.length > 0}>
         <summary className="cursor-pointer text-h6 font-semibold uppercase tracking-wider text-term-text">{t("tariff.tiersAdvanced")}</summary>
         <p className="hint mt-1.5 mb-2">{t("tariff.tiersHint")}</p>
+        {/* A stepped ("up-to") base rate cannot be combined with time tiers — the
+            engine would ignore them. Warn up-front; publishing is also blocked server-side. */}
+        {form.base.mode === "stepped" && form.tiers.length > 0 && (
+          <p className="mb-3 rounded-term border border-term-red/50 bg-term-red/10 px-3 py-2 text-[12px] text-term-red">
+            {t("tariff.steppedTiersConflict")}
+          </p>
+        )}
         {form.tiers.map((tr, i) => (
           <fieldset key={i} className="card mb-3 p-4">
             <legend className="flex items-center gap-2 px-1">