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/stores/firebaseAuthStore.ts

@@ -42,6 +42,11 @@ type AccessBillingPortalResponse =
   operations['AccessBillingPortal']['responses']['200']['content']['application/json']
 type AccessBillingPortalReqBody =
   operations['AccessBillingPortal']['requestBody']
+export type BillingPortalTargetTier = NonNullable<
+  NonNullable<
+    NonNullable<AccessBillingPortalReqBody>['content']
+  >['application/json']
+>['target_tier']
 
 export class FirebaseAuthStoreError extends Error {
   constructor(message: string) {
@@ -409,13 +414,15 @@ export const useFirebaseAuthStore = defineStore('firebaseAuth', () => {
     executeAuthAction((_) => addCredits(requestBodyContent))
 
   const accessBillingPortal = async (
-    requestBody?: AccessBillingPortalReqBody
+    targetTier?: BillingPortalTargetTier
   ): Promise<AccessBillingPortalResponse> => {
     const authHeader = await getAuthHeader()
     if (!authHeader) {
       throw new FirebaseAuthStoreError(t('toastMessages.userNotAuthenticated'))
     }
 
+    const requestBody = targetTier ? { target_tier: targetTier } : undefined
+
     const response = await fetch(buildApiUrl('/customers/billing'), {
       method: 'POST',
       headers: {