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.test.ts

@@ -8,8 +8,8 @@ import WorkspaceAuthGate from './WorkspaceAuthGate.vue'
 const mockIsInitialized = ref(false)
 const mockCurrentUser = ref<object | null>(null)
 
-vi.mock('@/stores/firebaseAuthStore', () => ({
-  useFirebaseAuthStore: () => ({
+vi.mock('@/stores/authStore', () => ({
+  useAuthStore: () => ({
     isInitialized: mockIsInitialized,
     currentUser: mockCurrentUser
   })