fix: use getAuthHeader for API key auth in subscription/billing (#9142)

## Summary

Fix "User not authenticated" errors when API key users
(desktop/portable) trigger subscription status checks or billing
operations.

## Changes

- **What**: Replace `getFirebaseAuthHeader()` with `getAuthHeader()` in
subscription and billing call sites (`fetchSubscriptionStatus`,
`initiateSubscriptionCheckout`, `fetchBalance`, `addCredits`,
`accessBillingPortal`, `performSubscriptionCheckout`). `getAuthHeader()`
supports the full auth fallback chain (workspace token → Firebase token
→ API key), whereas `getFirebaseAuthHeader()` returns null for API key
users since they bypass Firebase entirely. Also add an `isCloud` guard
to the subscription status watcher so non-cloud environments skip
subscription checks.

## Review Focus

- The `isCloud` guard on the watcher ensures local/desktop users never
hit the subscription endpoint. This was the originally intended design
per code owner confirmation.
- `getAuthHeader()` already exists in `firebaseAuthStore` with proper
fallback logic — no new auth code was added.

Fixes
https://www.notion.so/comfy-org/Bug-Subscription-status-check-occurring-in-non-cloud-environments-causing-authentication-errors-3116d73d365081738b21db157e88a9ed

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-9142-fix-use-getAuthHeader-for-API-key-auth-in-subscription-billing-3116d73d3650817fa345deaddc8c3fcd)
by [Unito](https://www.unito.io)

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

@@ -39,7 +39,7 @@ vi.mock('@/platform/telemetry', () => ({
 vi.mock('@/stores/firebaseAuthStore', () => ({
   useFirebaseAuthStore: vi.fn(() =>
     reactive({
-      getFirebaseAuthHeader: mockGetAuthHeader,
+      getAuthHeader: mockGetAuthHeader,
       userId: computed(() => mockUserId.value)
     })
   ),

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

@@ -54,7 +54,7 @@ export async function performSubscriptionCheckout(
   const firebaseAuthStore = useFirebaseAuthStore()
   const { userId } = storeToRefs(firebaseAuthStore)
   const telemetry = useTelemetry()
-  const authHeader = await firebaseAuthStore.getFirebaseAuthHeader()
+  const authHeader = await firebaseAuthStore.getAuthHeader()
 
   if (!authHeader) {
     throw new FirebaseAuthStoreError(t('toastMessages.userNotAuthenticated'))