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/app/globals.css

@@ -60,9 +60,11 @@
   padding-inline: 1rem;
 }
 
-/* Latin runs (model names, prices in EN digits) inside Farsi copy. */
+/* Latin runs (model names, SKUs) inside Farsi copy — Montserrat keeps
+   real 0-9 digits; IRANYekan fanum would otherwise paint them as ۰-۹. */
 .en {
   font-family: var(--font-en), var(--font-fa), sans-serif;
+  unicode-bidi: isolate;
 }
 
 /* Swiper theming: its own CSS reads these custom properties when drawing