feat(tariff): Tariff Lab — pure session-pricing simulator

Test rates "in time" (overnight windows, daily caps, overstay) in seconds against
any tariff version, instead of waiting hours/days. No real ledger writes.

- Extract priceSession() into @parking/shared: the grace/overstay wrapper over
  computeFee (unpaid -> entry..now; within-grace -> settled 0; grace-expired ->
  overstay, a fresh period from grace-expiry). PayStation.quote() now calls it so
  the booth and the lab can never diverge.
- API (tariffs.ts, tariff:read, read-only): POST /api/tariff/simulate prices a
  hypothetical session (active/any version/inline structure) and returns the
  priceSession outcome + a 30m..3d duration curve (see where the daily cap flattens);
  GET /api/tariff/simulate/session/:identity prefills from a real ledger session.
- UI TariffLab.tsx at Setup -> "Tariff Lab": version picker, entry/asOf times,
  optional payment+grace, category, and load-a-real-ticket. Admin-gated, available
  on-site (useful to quote a dispute).
- 4 new priceSession unit tests incl. the ticket-1245791632490 overstay-not-zero
  regression (40 pass). i18n lab.* + nav.tariffLab (sq+en). Verified live via the UI.

Wiki: tariff (priceSession + Tariff Lab as-built), log.

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

apps/web/src/lib/i18n/sq.ts

@@ -46,6 +46,7 @@ export const sq = {
     setup: "Konfigurimi",
     devices: "Pajisjet",
     tariff: "Tarifa",
+    tariffLab: "Lab Tarife",
     subscriptions: "Abonimet",
     site: "Park",
     users: "Përdoruesit",
@@ -341,6 +342,36 @@ export const sq = {
     relayLabel: "Rele {{relay}} ({{direction}})",
     noRelaysConfigured: "Ky kontrollues nuk ka rele të konfiguruar.",
   },
+  lab: {
+    title: "Lab Tarife",
+    intro:
+      "Testo tarifat në kohë (dritare ditë/natë, kufi ditor, qëndrim tej afatit) në sekonda, pa pritur orë. Çmimi llogaritet me të njëjtën logjikë si kabina; nuk shkruhet asgjë në ledger.",
+    loadTicket: "Ngarko nga një biletë reale",
+    loadTicketPh: "Numri i biletës / identiteti",
+    load: "Ngarko",
+    loaded: "U ngarkua sesioni {{id}}",
+    tariffVersion: "Versioni i tarifës",
+    activeVersion: "Versioni aktiv (i tanishëm)",
+    entered: "Hyrja",
+    asOf: "Deri më (tani/dalja)",
+    now: "Tani",
+    category: "Kategoria",
+    categoryPh: "p.sh. bus (bosh = makinë)",
+    payment: "Pagesa",
+    paid: "u pagua",
+    graceMin: "afati (min)",
+    price: "Llogarit çmimin",
+    pricing: "Duke llogaritur…",
+    outcome: "Rezultati",
+    amountDue: "Shuma për pagesë",
+    billedPeriod: "Periudha e faturuar",
+    overstay: "tej afatit",
+    settled: "i shlyer",
+    periodStart: "Fillimi i periudhës",
+    graceExpires: "Afati skadon",
+    curve: "Kurba sipas kohëzgjatjes",
+    curveHint: "Tarifa nga hyrja për disa kohëzgjatje — shih ku rrafshohet kufiri ditor ose ndryshojnë dritaret.",
+  },
   subs: {
     title: "Abonimet",
     unnamed: "(pa emër)",