Add installment calculator and Latin digits in English product text.

Wire the product CTA to store-loan-methods calculate so shoppers can pick a term and see the plan, and render English model runs under Montserrat so fanum fonts stop painting 0-9 as Persian digits.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/lib/loanMethods.ts

@@ -0,0 +1,53 @@
+import { API_BASE, SITE_DOMAIN } from "./config";
+import type { StoreLoanCalculateResponse, StoreLoanMethodsResponse } from "./types";
+
+const base = `${API_BASE}/tenants/${SITE_DOMAIN}/store-loan-methods`;
+
+/** Browser-side list — same contract as GET .../store-loan-methods. */
+export async function fetchStoreLoanMethods(
+  signal?: AbortSignal,
+): Promise<StoreLoanMethodsResponse> {
+  const res = await fetch(base, { signal });
+  if (!res.ok) throw new Error(`loan methods failed (${res.status})`);
+  return res.json() as Promise<StoreLoanMethodsResponse>;
+}
+
+/** Browser-side calculate — POST .../store-loan-methods/calculate. */
+export async function fetchStoreLoanPlan(
+  input: {
+    methodId: string;
+    totalValue: number;
+    creditAmount: number;
+    months: number;
+  },
+  signal?: AbortSignal,
+): Promise<StoreLoanCalculateResponse> {
+  const res = await fetch(`${base}/calculate`, {
+    method: "POST",
+    headers: { "Content-Type": "application/json" },
+    body: JSON.stringify(input),
+    signal,
+  });
+  if (!res.ok) {
+    let message = `calculate failed (${res.status})`;
+    try {
+      const body = (await res.json()) as { message?: string };
+      if (body.message) message = body.message;
+    } catch {
+      /* ignore */
+    }
+    throw new Error(message);
+  }
+  return res.json() as Promise<StoreLoanCalculateResponse>;
+}
+
+export function clampCreditAmount(
+  value: number,
+  totalValue: number,
+  minAmount: number,
+  maxAmount: number,
+): number {
+  const upper = Math.min(Math.round(maxAmount), Math.round(totalValue));
+  const lower = Math.min(Math.round(minAmount), upper);
+  return Math.min(upper, Math.max(lower, Math.round(value)));
+}