refactor: rename firebaseAuthStore to authStore with shared test fixtures

- Rename src/stores/firebaseAuthStore.ts → src/stores/authStore.ts
- Rename src/composables/auth/useFirebaseAuthActions.ts → src/composables/auth/useAuthActions.ts
- Rename exports: useFirebaseAuthStore → useAuthStore, FirebaseAuthStoreError → AuthStoreError
- Update store ID from 'firebaseAuth' to 'auth'
- Create shared mock factory at src/stores/__tests__/authStoreMock.ts
- Update 27 production import sites and 16 test files

Fixes #8219

src/platform/cloud/subscription/components/SubscriptionPanel.test.ts

@@ -82,8 +82,8 @@ vi.mock(
   })
 )
 
-vi.mock('@/composables/auth/useFirebaseAuthActions', () => ({
-  useFirebaseAuthActions: vi.fn(() => ({
+vi.mock('@/composables/auth/useAuthActions', () => ({
+  useAuthActions: vi.fn(() => ({
     authActions: vi.fn(() => ({
       accessBillingPortal: vi.fn()
     }))