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/platform/cloud/subscription/components/SubscriptionPanel.vue

@@ -121,7 +121,7 @@
                       height="2rem"
                     />
                     <div v-else class="text-2xl font-bold">
-                      ${{ totalCredits }}
+                      {{ totalCredits }}
                     </div>
                   </div>
 
@@ -134,7 +134,7 @@
                         height="1rem"
                       />
                       <div v-else class="text-sm text-text-secondary font-bold">
-                        ${{ monthlyBonusCredits }}
+                        {{ monthlyBonusCredits }}
                       </div>
                       <div class="flex items-center gap-1">
                         <div class="text-sm text-text-secondary">
@@ -162,7 +162,7 @@
                         height="1rem"
                       />
                       <div v-else class="text-sm text-text-secondary font-bold">
-                        ${{ prepaidCredits }}
+                        {{ prepaidCredits }}
                       </div>
                       <div class="flex items-center gap-1">
                         <div class="text-sm text-text-secondary">

src/platform/cloud/subscription/composables/useSubscriptionCredits.ts

@@ -1,58 +1,37 @@
 import { computed } from 'vue'
+import { useI18n } from 'vue-i18n'
 
+import { formatCreditsFromCents } from '@/base/credits/comfyCredits'
 import { useFirebaseAuthStore } from '@/stores/firebaseAuthStore'
-import { formatMetronomeCurrency } from '@/utils/formatUtil'
 
 /**
  * Composable for handling subscription credit calculations and formatting
  */
 export function useSubscriptionCredits() {
   const authStore = useFirebaseAuthStore()
+  const { t, locale } = useI18n()
 
-  const totalCredits = computed(() => {
-    if (!authStore.balance?.amount_micros) return '0.00'
-    try {
-      return formatMetronomeCurrency(authStore.balance.amount_micros, 'usd')
-    } catch (error) {
-      console.error(
-        '[useSubscriptionCredits] Error formatting total credits:',
-        error
-      )
-      return '0.00'
-    }
-  })
+  const formatBalance = (maybeCents?: number) => {
+    // Backend returns cents despite the *_micros naming convention.
+    const cents = maybeCents ?? 0
+    const amount = formatCreditsFromCents({
+      cents,
+      locale: locale.value
+    })
+    return `${amount} ${t('credits.credits')}`
+  }
 
-  const monthlyBonusCredits = computed(() => {
-    if (!authStore.balance?.cloud_credit_balance_micros) return '0.00'
-    try {
-      return formatMetronomeCurrency(
-        authStore.balance.cloud_credit_balance_micros,
-        'usd'
-      )
-    } catch (error) {
-      console.error(
-        '[useSubscriptionCredits] Error formatting monthly bonus credits:',
-        error
-      )
-      return '0.00'
-    }
-  })
+  const totalCredits = computed(() =>
+    formatBalance(authStore.balance?.amount_micros)
+  )
 
-  const prepaidCredits = computed(() => {
-    if (!authStore.balance?.prepaid_balance_micros) return '0.00'
-    try {
-      return formatMetronomeCurrency(
-        authStore.balance.prepaid_balance_micros,
-        'usd'
-      )
-    } catch (error) {
-      console.error(
-        '[useSubscriptionCredits] Error formatting prepaid credits:',
-        error
-      )
-      return '0.00'
-    }
-  })
+  const monthlyBonusCredits = computed(() =>
+    formatBalance(authStore.balance?.cloud_credit_balance_micros)
+  )
+
+  const prepaidCredits = computed(() =>
+    formatBalance(authStore.balance?.prepaid_balance_micros)
+  )
 
   const isLoadingBalance = computed(() => authStore.isFetchingBalance)