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/lib/i18n/sq.ts

@@ -246,6 +246,8 @@ export const sq = {
     stepUpTo: "Deri në",
     stepTotal: "Çmimi total",
     addStep: "+ Shto rresht",
+    steppedTiersConflict:
+      "⚠ Nivelet kohore/sezonale NUK zbatohen kur tarifa bazë është 'Sipas kohëzgjatjes (deri-në)' — motori i shpërfill plotësisht. Hiqi nivelet, ose ndrysho tarifën bazë në 'Shkallë orësh' a 'Çmim fiks'. Publikimi bllokohet derisa kjo të rregullohet.",
     tiersAdvanced: "Të avancuara: nivele kohore & sezonale",
     tiersHint: "Opsionale. Shto nivele tarifore që vlejnë vetëm në orë/ditë/data ose kategori të caktuara (p.sh. orë e lirë, tarifë nate, fundjavë, autobus). Pa nivele, publikohet vetëm tarifa bazë.",
     tierName: "Emri",