feat: billing brl pix new users (#14617)

## Linear ticket
- https://linear.app/chatwoot/issue/CW-7253/billing-brl-pix-new-users

## Description

New accounts that sign up in Brazilian Portuguese are now billed in BRL
instead of USD. Their Stripe customer is created with a Brazil address
and Portuguese locale (so the Stripe portal offers Real prices and PIX),
and the AI credit top-up flow shows packages priced in the account's
billing currency. Currency support is config-driven, so adding another
currency later is a configuration change rather than a code change.


## Type of change

- [ ] New feature (non-breaking change which adds functionality)

## How Has This Been Tested?

- https://www.loom.com/share/c8d3d08c1b844ed6b820438d4209491a

## Screenshot
<img width="904" height="440" alt="image"
src="https://github.com/user-attachments/assets/6f19fad8-e6af-46ea-b99f-b0265bb9eeec"
/>


## Checklist:

- [ ] My code follows the style guidelines of this project
- [ ] I have performed a self-review of my code
- [ ] I have commented on my code, particularly in hard-to-understand
areas
- [ ] I have made corresponding changes to the documentation
- [ ] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [ ] New and existing unit tests pass locally with my changes
- [ ] Any dependent changes have been merged and published in downstream
modules

app/javascript/dashboard/constants/billing.js

@@ -0,0 +1,35 @@
+// Single source of truth for billing currencies on the frontend.
+// Adding a currency = one entry in BILLING_CURRENCY_CONFIG, add the code to
+// SUPPORTED_BILLING_CURRENCIES, and add its label key under
+// BILLING_SETTINGS.CURRENCY.OPTIONS in the locale files.
+
+export const DEFAULT_BILLING_CURRENCY = 'usd';
+
+// Order here drives the order of the currency toggle in the UI.
+export const SUPPORTED_BILLING_CURRENCIES = ['usd', 'brl'];
+
+export const BILLING_CURRENCY_CONFIG = {
+  usd: {
+    code: 'usd',
+    intlLocale: 'en-US',
+    i18nLabelKey: 'BILLING_SETTINGS.CURRENCY.OPTIONS.USD',
+  },
+  brl: {
+    code: 'brl',
+    intlLocale: 'pt-BR',
+    i18nLabelKey: 'BILLING_SETTINGS.CURRENCY.OPTIONS.BRL',
+  },
+};
+
+export const getCurrencyConfig = code =>
+  BILLING_CURRENCY_CONFIG[(code || DEFAULT_BILLING_CURRENCY).toLowerCase()] ||
+  BILLING_CURRENCY_CONFIG[DEFAULT_BILLING_CURRENCY];
+
+export const formatCurrencyAmount = (amount, code, options = {}) => {
+  const { intlLocale, code: currencyCode } = getCurrencyConfig(code);
+  return new Intl.NumberFormat(intlLocale, {
+    style: 'currency',
+    currency: currencyCode.toUpperCase(),
+    ...options,
+  }).format(amount);
+};