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/components/PricingTable.test.ts

@@ -16,7 +16,7 @@ const mockAccessBillingPortal = vi.fn()
 const mockReportError = vi.fn()
 const mockTrackBeginCheckout = vi.fn()
 const mockUserId = ref<string | undefined>('user-123')
-const mockGetFirebaseAuthHeader = vi.fn(() =>
+const mockGetAuthHeader = vi.fn(() =>
   Promise.resolve({ Authorization: 'Bearer test-token' })
 )
 const mockGetCheckoutAttribution = vi.hoisted(() => vi.fn(() => ({})))
@@ -58,7 +58,7 @@ vi.mock('@/composables/useErrorHandling', () => ({
 vi.mock('@/stores/firebaseAuthStore', () => ({
   useFirebaseAuthStore: () =>
     reactive({
-      getFirebaseAuthHeader: mockGetFirebaseAuthHeader,
+      getAuthHeader: mockGetAuthHeader,
       userId: computed(() => mockUserId.value)
     }),
   FirebaseAuthStoreError: class extends Error {}