feat: pass target tier to billing portal for subscription updates (#7692)

## Summary

Pass target tier to billing portal API for deep linking to Stripe's
subscription update confirmation screen when user has an active
subscription.

## Changes

- **What**: When a user with an active subscription clicks a tier in
PricingTable, pass the target tier (including billing cycle) to
`accessBillingPortal` which sends it as `target_tier` in the request
body. This enables the backend to create a Stripe billing portal deep
link directly to the subscription update confirmation screen.
- **Dependencies**: Requires comfy-api PR for `POST /customers/billing`
`target_tier` support

## Review Focus

- PricingTable now differentiates between new subscriptions (checkout
flow) and existing subscriptions (billing portal with deep link)
- Type derivation uses `Parameters<typeof
authStore.accessBillingPortal>[0]` to avoid duplicating the tier union
(matches codebase pattern)
- Registry types manually updated to include `target_tier` field (will
be regenerated when API is deployed)

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-7692-feat-pass-target-tier-to-billing-portal-for-subscription-updates-2d06d73d365081b38fe4c81e95dce58c)
by [Unito](https://www.unito.io)

---------

Co-authored-by: Christian Byrne <cbyrne@comfy.org>
Co-authored-by: GitHub Action <action@github.com>

src/platform/cloud/subscription/components/PricingTable.vue

@@ -333,7 +333,7 @@ const { n } = useI18n()
 const { getAuthHeader } = useFirebaseAuthStore()
 const { isActiveSubscription, subscriptionTier, isYearlySubscription } =
   useSubscription()
-const { reportError } = useFirebaseAuthActions()
+const { accessBillingPortal, reportError } = useFirebaseAuthActions()
 const { wrapWithErrorHandlingAsync } = useErrorHandling()
 
 const isLoading = ref(false)
@@ -443,9 +443,15 @@ const handleSubscribe = wrapWithErrorHandlingAsync(
     loadingTier.value = tierKey
 
     try {
-      const response = await initiateCheckout(tierKey)
-      if (response.checkout_url) {
-        window.open(response.checkout_url, '_blank')
+      if (isActiveSubscription.value) {
+        // Pass the target tier to create a deep link to subscription update confirmation
+        const checkoutTier = getCheckoutTier(tierKey, currentBillingCycle.value)
+        await accessBillingPortal(checkoutTier)
+      } else {
+        const response = await initiateCheckout(tierKey)
+        if (response.checkout_url) {
+          window.open(response.checkout_url, '_blank')
+        }
       }
     } finally {
       isLoading.value = false