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>

packages/registry-types/src/comfyRegistryTypes.ts

@@ -11910,6 +11910,8 @@ export interface operations {
                 "application/json": {
                     /** @description Optional URL to redirect the customer after they're done with the billing portal */
                     return_url?: string;
+                    /** @description Optional target subscription tier. When provided, creates a deep link directly to the subscription update confirmation screen with this tier pre-selected. */
+                    target_tier?: "standard" | "creator" | "pro" | "standard-yearly" | "creator-yearly" | "pro-yearly";
                 };
             };
         };