feat: support effective_balance_micros for user balance display (#7658)

## Summary

Add support for the new `effective_balance_micros` field to show users
their effective balance accounting for pending charges.

## Changes

- **What**: Update balance display components to use
`effective_balance_micros` (with fallback to `amount_micros` for
backwards compatibility)
- **Types**: Add `pending_charges_micros` and `effective_balance_micros`
to `GetCustomerBalance` response type in registry-types

## Review Focus

- The fallback pattern ensures backwards compatibility if the API
doesn't return the new field
- The `effective_balance_micros` can be negative when pending charges
exceed the available balance

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-7658-feat-support-effective_balance_micros-for-user-balance-display-2cf6d73d36508193a5a7e999f3185078)
by [Unito](https://www.unito.io)

src/components/common/UserCredit.vue

@@ -42,8 +42,10 @@ const balanceLoading = computed(() => authStore.isFetchingBalance)
 const { t, locale } = useI18n()
 
 const formattedBalance = computed(() => {
-  // Backend returns cents despite the *_micros naming convention.
-  const cents = authStore.balance?.amount_micros ?? 0
+  const cents =
+    authStore.balance?.effective_balance_micros ??
+    authStore.balance?.amount_micros ??
+    0
   const amount = formatCreditsFromCents({
     cents,
     locale: locale.value
@@ -52,8 +54,10 @@ const formattedBalance = computed(() => {
 })
 
 const formattedCreditsOnly = computed(() => {
-  // Backend returns cents despite the *_micros naming convention.
-  const cents = authStore.balance?.amount_micros ?? 0
+  const cents =
+    authStore.balance?.effective_balance_micros ??
+    authStore.balance?.amount_micros ??
+    0
   const amount = formatCreditsFromCents({
     cents,
     locale: locale.value,