add shared comfy credit conversion helpers (#7061)

Introduces cents<->usd<->credit converters plus basic formatters and
adds test. Lays groundwork to start converting UI components into
displaying comfy credits.

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-7061-add-shared-comfy-credit-conversion-helpers-2bb6d73d3650810bb34fdf9bb3fc115b)
by [Unito](https://www.unito.io)

---------

Co-authored-by: Claude <noreply@anthropic.com>

src/components/common/UserCredit.vue

@@ -26,10 +26,11 @@
 import Skeleton from 'primevue/skeleton'
 import Tag from 'primevue/tag'
 import { computed } from 'vue'
+import { useI18n } from 'vue-i18n'
 
+import { formatCreditsFromCents } from '@/base/credits/comfyCredits'
 import { useFeatureFlags } from '@/composables/useFeatureFlags'
 import { useFirebaseAuthStore } from '@/stores/firebaseAuthStore'
-import { formatMetronomeCurrency } from '@/utils/formatUtil'
 
 const { textClass } = defineProps<{
   textClass?: string
@@ -38,9 +39,15 @@ const { textClass } = defineProps<{
 const authStore = useFirebaseAuthStore()
 const { flags } = useFeatureFlags()
 const balanceLoading = computed(() => authStore.isFetchingBalance)
+const { t, locale } = useI18n()
 
 const formattedBalance = computed(() => {
-  if (!authStore.balance) return '0.00'
-  return formatMetronomeCurrency(authStore.balance.amount_micros, 'usd')
+  // Backend returns cents despite the *_micros naming convention.
+  const cents = authStore.balance?.amount_micros ?? 0
+  const amount = formatCreditsFromCents({
+    cents,
+    locale: locale.value
+  })
+  return `${amount} ${t('credits.credits')}`
 })
 </script>