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/composables/auth/useFirebaseAuthActions.ts

@@ -11,6 +11,7 @@ import { useTelemetry } from '@/platform/telemetry'
 import { useToastStore } from '@/platform/updates/common/toastStore'
 import { useDialogService } from '@/services/dialogService'
 import { useFirebaseAuthStore } from '@/stores/firebaseAuthStore'
+import type { BillingPortalTargetTier } from '@/stores/firebaseAuthStore'
 import { usdToMicros } from '@/utils/formatUtil'
 
 /**
@@ -102,8 +103,11 @@ export const useFirebaseAuthActions = () => {
     window.open(response.checkout_url, '_blank')
   }, reportError)
 
-  const accessBillingPortal = wrapWithErrorHandlingAsync(async () => {
-    const response = await authStore.accessBillingPortal()
+  const accessBillingPortal = wrapWithErrorHandlingAsync<
+    [targetTier?: BillingPortalTargetTier],
+    void
+  >(async (targetTier) => {
+    const response = await authStore.accessBillingPortal(targetTier)
     if (!response.billing_portal_url) {
       throw new Error(
         t('toastMessages.failedToAccessBillingPortal', {