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/api.ts

@@ -29,7 +29,7 @@ export async function apiFetch<T>(path: string, init: RequestInit = {}): Promise
   }
   const res = await fetch(path, { ...init, headers, credentials: "include" });
   if (!res.ok) {
-    const msg = (await res.json().catch(() => ({}))) as { error?: string };
+    const msg = (await res.json().catch(() => ({}))) as { error?: string; problems?: string[] };
     const error = msg.error ?? `${path}: ${res.status}`;
     // Ship the failed request to the backend log store (best-effort, loop-safe — the
     // logger itself never logs the /api/logs call). 401s are normal pre-login churn,
@@ -37,7 +37,7 @@ export async function apiFetch<T>(path: string, init: RequestInit = {}): Promise
     if (res.status !== 401) {
       logFailedRequest({ path, method, status: res.status, error });
     }
-    throw new ApiError(error, res.status);
+    throw new ApiError(error, res.status, msg.problems);
   }
   if (res.status === 204) return undefined as T;
   return res.json() as Promise<T>;
@@ -47,6 +47,8 @@ export class ApiError extends Error {
   constructor(
     message: string,
     readonly status: number,
+    /** Field-level problems from a validation error (e.g. tariff publish), if any. */
+    readonly problems?: string[],
   ) {
     super(message);
   }