feat: add Firebase client SDK setup

- Install firebase package
- Add environment variable configuration (.env.example)
- Create firebase.ts module that initializes Firebase app, Firestore, Auth, and Storage
- Add comprehensive test coverage for Firebase initialization with mocked Firebase modules
- Update .gitignore to exclude local environment files

src/lib/firebase.test.ts

@@ -0,0 +1,42 @@
+import { describe, it, expect, vi, beforeEach } from 'vitest'
+
+const initializeAppMock = vi.fn(() => ({ name: 'mock-app' }))
+const getFirestoreMock = vi.fn(() => ({ type: 'mock-firestore' }))
+const getAuthMock = vi.fn(() => ({ type: 'mock-auth' }))
+const getStorageMock = vi.fn(() => ({ type: 'mock-storage' }))
+
+vi.mock('firebase/app', () => ({ initializeApp: initializeAppMock }))
+vi.mock('firebase/firestore', () => ({ getFirestore: getFirestoreMock }))
+vi.mock('firebase/auth', () => ({ getAuth: getAuthMock }))
+vi.mock('firebase/storage', () => ({ getStorage: getStorageMock }))
+
+describe('firebase client setup', () => {
+  beforeEach(() => {
+    vi.resetModules()
+    initializeAppMock.mockClear()
+  })
+
+  it('initializes the app with config read from Vite env vars and exports db/auth/storage', async () => {
+    vi.stubEnv('VITE_FIREBASE_API_KEY', 'test-api-key')
+    vi.stubEnv('VITE_FIREBASE_AUTH_DOMAIN', 'test.firebaseapp.com')
+    vi.stubEnv('VITE_FIREBASE_PROJECT_ID', 'test-project')
+    vi.stubEnv('VITE_FIREBASE_STORAGE_BUCKET', 'test-project.appspot.com')
+    vi.stubEnv('VITE_FIREBASE_MESSAGING_SENDER_ID', '123')
+    vi.stubEnv('VITE_FIREBASE_APP_ID', 'app-id')
+
+    const { app, db, auth, storage } = await import('./firebase')
+
+    expect(initializeAppMock).toHaveBeenCalledWith({
+      apiKey: 'test-api-key',
+      authDomain: 'test.firebaseapp.com',
+      projectId: 'test-project',
+      storageBucket: 'test-project.appspot.com',
+      messagingSenderId: '123',
+      appId: 'app-id',
+    })
+    expect(app).toEqual({ name: 'mock-app' })
+    expect(db).toEqual({ type: 'mock-firestore' })
+    expect(auth).toEqual({ type: 'mock-auth' })
+    expect(storage).toEqual({ type: 'mock-storage' })
+  })
+})

src/lib/firebase.ts

@@ -0,0 +1,18 @@
+import { initializeApp } from 'firebase/app'
+import { getFirestore } from 'firebase/firestore'
+import { getAuth } from 'firebase/auth'
+import { getStorage } from 'firebase/storage'
+
+const firebaseConfig = {
+  apiKey: import.meta.env.VITE_FIREBASE_API_KEY,
+  authDomain: import.meta.env.VITE_FIREBASE_AUTH_DOMAIN,
+  projectId: import.meta.env.VITE_FIREBASE_PROJECT_ID,
+  storageBucket: import.meta.env.VITE_FIREBASE_STORAGE_BUCKET,
+  messagingSenderId: import.meta.env.VITE_FIREBASE_MESSAGING_SENDER_ID,
+  appId: import.meta.env.VITE_FIREBASE_APP_ID,
+}
+
+export const app = initializeApp(firebaseConfig)
+export const db = getFirestore(app)
+export const auth = getAuth(app)
+export const storage = getStorage(app)