refactor: rename firebaseAuthStore to authStore with shared test fixtures (#10483)

## Summary

Rename `useFirebaseAuthStore` → `useAuthStore` and
`FirebaseAuthStoreError` → `AuthStoreError`. Introduce shared mock
factory (`authStoreMock.ts`) to replace 16 independent bespoke mocks.

## Changes

- **What**: Mechanical rename of store, composable, class, and store ID
(`firebaseAuth` → `auth`). Created
`src/stores/__tests__/authStoreMock.ts` — a shared mock factory with
reactive controls, used by all consuming test files. Migrated all 16
test files from ad-hoc mocks to the shared factory.
- **Files**: 62 files changed (rename propagation + new test infra)

## Review Focus

- Mock factory API design in `authStoreMock.ts` — covers all store
properties with reactive `controls` for per-test customization
- Self-test in `authStoreMock.test.ts` validates computed reactivity

Fixes #8219

## Stack

This is PR 1/5 in a stacked refactoring series:
1. **→ This PR**: Rename + shared test fixtures
2. #10484: Extract auth-routing from workspaceApi
3. #10485: Auth token priority tests
4. #10486: Decompose MembersPanelContent
5. #10487: Consolidate SubscriptionTier type

---------

Co-authored-by: Alexander Brown <drjkl@comfy.org>

src/platform/workspace/auth/WorkspaceAuthGate.vue

@@ -27,7 +27,7 @@ import { isCloud } from '@/platform/distribution/types'
 import { useSubscriptionDialog } from '@/platform/cloud/subscription/composables/useSubscriptionDialog'
 import { refreshRemoteConfig } from '@/platform/remoteConfig/refreshRemoteConfig'
 import { useTeamWorkspaceStore } from '@/platform/workspace/stores/teamWorkspaceStore'
-import { useFirebaseAuthStore } from '@/stores/firebaseAuthStore'
+import { useAuthStore } from '@/stores/authStore'
 
 const FIREBASE_INIT_TIMEOUT_MS = 16_000
 const CONFIG_REFRESH_TIMEOUT_MS = 10_000
@@ -38,7 +38,7 @@ const subscriptionDialog = useSubscriptionDialog()
 async function initialize(): Promise<void> {
   if (!isCloud) return
 
-  const authStore = useFirebaseAuthStore()
+  const authStore = useAuthStore()
   const { isInitialized, currentUser } = storeToRefs(authStore)
 
   try {