feat: handling subscription tier button link parameter (#7553)

## Summary

Discussion here:
https://comfy-organization.slack.com/archives/C0A0XANFJRE/p1764899027465379

Implement: Subscription tier query parameter for direct checkout flow

Example button link: `/cloud/subscribe?tier=standard`

`tier` could be `standard`, `creator` or `pro`
`cycle` could be `monthly` or `yearly`. it is optional, and `monthly` by
default.

<!-- One sentence describing what changed and why. -->

## Changes

- **What**: <!-- Core functionality added/modified -->
- Add a landing page called `CloudSubscriptionRedirectView.vue` to
handling the subscription tier button link parameter
  - Extract subscription handling logic from `PriceTable.vue`
 
- **Breaking**: <!-- Any breaking changes (if none, remove this line)
-->
  - Code change touched  `PriceTable.vue`
- **Dependencies**: <!-- New dependencies (if none, remove this line)
-->

## Review Focus
- link will redirect to login url, when cloud app not login
- after login, the cloud app will redirect to CloudSubscriptionRedirect
page
- wait for several seconds, the cloud app will be redirected to checkout
page

<!-- Critical design decisions or edge cases that need attention -->

<!-- If this PR fixes an issue, uncomment and update the line below -->
<!-- Fixes #ISSUE_NUMBER -->

## Screenshots (if applicable)

![Kapture 2025-12-16 at 18 43
28](https://github.com/user-attachments/assets/affbc18f-d45c-4953-b06a-fc797eba6804)


<!-- Add screenshots or video recording to help explain your changes -->

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-7553-feat-handling-subscription-tier-button-link-parameter-2cb6d73d365081ee9580e89090248300)
by [Unito](https://www.unito.io)

---------

Co-authored-by: GitHub Action <action@github.com>

src/platform/cloud/subscription/utils/subscriptionCheckoutUtil.ts

@@ -0,0 +1,87 @@
+import { getComfyApiBaseUrl } from '@/config/comfyApi'
+import { t } from '@/i18n'
+import { isCloud } from '@/platform/distribution/types'
+import {
+  FirebaseAuthStoreError,
+  useFirebaseAuthStore
+} from '@/stores/firebaseAuthStore'
+import type { TierKey } from '@/platform/cloud/subscription/constants/tierPricing'
+import type { BillingCycle } from './subscriptionTierRank'
+
+type CheckoutTier = TierKey | `${TierKey}-yearly`
+
+const getCheckoutTier = (
+  tierKey: TierKey,
+  billingCycle: BillingCycle
+): CheckoutTier => (billingCycle === 'yearly' ? `${tierKey}-yearly` : tierKey)
+
+/**
+ * Core subscription checkout logic shared between PricingTable and
+ * SubscriptionRedirectView. Handles:
+ * - Ensuring the user is authenticated
+ * - Calling the backend checkout endpoint
+ * - Normalizing error responses
+ * - Opening the checkout URL in a new tab when available
+ *
+ * Callers are responsible for:
+ * - Guarding on cloud-only behavior (isCloud)
+ * - Managing loading state
+ * - Wrapping with error handling (e.g. useErrorHandling)
+ */
+export async function performSubscriptionCheckout(
+  tierKey: TierKey,
+  currentBillingCycle: BillingCycle,
+  openInNewTab: boolean = true
+): Promise<void> {
+  if (!isCloud) return
+
+  const { getAuthHeader } = useFirebaseAuthStore()
+  const authHeader = await getAuthHeader()
+
+  if (!authHeader) {
+    throw new FirebaseAuthStoreError(t('toastMessages.userNotAuthenticated'))
+  }
+
+  const checkoutTier = getCheckoutTier(tierKey, currentBillingCycle)
+
+  const response = await fetch(
+    `${getComfyApiBaseUrl()}/customers/cloud-subscription-checkout/${checkoutTier}`,
+    {
+      method: 'POST',
+      headers: { ...authHeader, 'Content-Type': 'application/json' }
+    }
+  )
+
+  if (!response.ok) {
+    let errorMessage = 'Failed to initiate checkout'
+    try {
+      const errorData = await response.json()
+      errorMessage = errorData.message || errorMessage
+    } catch {
+      // If JSON parsing fails, try to get text response or use HTTP status
+      try {
+        const errorText = await response.text()
+        errorMessage =
+          errorText || `HTTP ${response.status} ${response.statusText}`
+      } catch {
+        errorMessage = `HTTP ${response.status} ${response.statusText}`
+      }
+    }
+
+    throw new FirebaseAuthStoreError(
+      t('toastMessages.failedToInitiateSubscription', {
+        error: errorMessage
+      })
+    )
+  }
+
+  const data = await response.json()
+
+  if (data.checkout_url) {
+    if (openInNewTab) {
+      window.open(data.checkout_url, '_blank')
+    } else {
+      globalThis.location.href = data.checkout_url
+    }
+  }
+}