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

packages/shared/src/tariff.test.ts

@@ -229,6 +229,19 @@ describe("validate V2", () => {
     const errs = validateTariffStructure({ ...base, defaultCard: { ...okDefault, window: { dow: [1] } } });
     expect(errs).toContain("defaultCard must not have a window (it is the always-active fallback)");
   });
+  it("rejects a STEPPED base card combined with windowed tiers (they would be ignored)", () => {
+    const steppedDefault: TariffCard = { name: "d", priority: 0, steps: [{ uptoMin: 60, totalMinor: 200 }] };
+    const errs = validateTariffStructure({
+      ...base,
+      defaultCard: steppedDefault,
+      windowedCards: [{ name: "night", priority: 1, window: { dow: [1] }, blocks: ladder(5000) }],
+    });
+    expect(errs.some((e) => /up-to-duration \(stepped\) base/.test(e))).toBe(true);
+  });
+  it("accepts a STEPPED base card with NO tiers", () => {
+    const steppedDefault: TariffCard = { name: "d", priority: 0, steps: [{ uptoMin: 60, totalMinor: 200 }] };
+    expect(validateTariffStructure({ ...base, defaultCard: steppedDefault })).toEqual([]);
+  });
   it("rejects a bad hour format", () => {
     const errs = validateTariffStructure({ ...base, defaultCard: okDefault, windowedCards: [{ name: "w", priority: 1, window: { fromHour: "25:00", toHour: "26:00" }, blocks: ladder(5000) }] });
     expect(errs.some((e) => e.includes("fromHour"))).toBe(true);