chore: remove Firebase backend code, replaced by Supabase (see 2026-08-08 spec)

Deleted all Firebase-specific files and directories:
- Configuration: .firebaserc, firebase.json, firestore.* files, storage.rules
- Cloud Functions: functions/ directory (package.json, source, config)
- Tests: firebase-tests/, vitest.rules.config.ts
- Scripts: scripts/seed.ts
- Source: src/lib/firebase.ts and .test.ts
- Environment: .env.example

Removed dependencies:
- firebase (main package)
- firebase-admin, firebase-tools (dev)
- @firebase/rules-unit-testing (dev)

Removed scripts:
- test:rules (Firebase emulator tests)
- seed (Firestore seed script)

Updated package.json via npm install (729 packages removed)
Updated .gitignore (removed functions/ entries)
Updated vite.config.ts (removed firebase-tests and functions exclusions)

All tests, builds, and lints pass cleanly. No Firebase references remain in
the codebase (only in historical spec documents in docs/).

Claude-Session: https://claude.ai/code/session_01NhxA4npAXAxmCDEJt6kxSH

src/lib/firebase.test.ts

@@ -1,42 +0,0 @@
-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

@@ -1,18 +0,0 @@
-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)