M2: refactor hooks to storage adapter (subscribe)

- src/storage/index.js: getStorage() factory + SDK re-exports
- App.js: useFirestoreDocument/Collection call storage.subscribeDoc/Collection
- getStorage import added

56 frontend tests green. Hooks now impl-agnostic (firebase vs ws).

src/storage/index.js

@@ -1,13 +1,47 @@
-// src/storage/index.js — barrel re-export of Firebase SDK.
-// Phase C: App.js swaps imports from 'firebase/*' to here.
-// STORAGE=firebase = identical behavior. Zero risk.
-// Later: factory picks impl based on REACT_APP_STORAGE env.
+// src/storage/index.js — storage factory + SDK re-exports.
+// STORAGE=firebase (default): adapter wraps SDK. STORAGE=ws: backend.
+// App.js imports getStorage() for subscribe; still imports SDK pieces for writes (per-group refactor pending).
 
-export { initializeApp } from 'firebase/app';
-export {
+import { initializeApp } from 'firebase/app';
+import {
   getAuth, signInAnonymously, onAuthStateChanged, signInWithCustomToken,
 } from 'firebase/auth';
-export {
+import {
   getFirestore, doc, setDoc, getDoc, getDocs, addDoc, collection,
   onSnapshot, updateDoc, deleteDoc, query, orderBy, limit, writeBatch,
 } from 'firebase/firestore';
+import { initFirebase, createFirebaseStorage } from './firebase';
+
+let storageInstance = null;
+
+// Returns adapter instance implementing interface (getDoc/setDoc/subscribeDoc/etc).
+export function getStorage() {
+  if (storageInstance) return storageInstance;
+  const mode = process.env.REACT_APP_STORAGE || 'firebase';
+  if (mode === 'firebase') {
+    const ok = initFirebase();
+    if (!ok) throw new Error('Firebase config missing. Check REACT_APP_FIREBASE_* env.');
+    storageInstance = createFirebaseStorage();
+  } else if (mode === 'ws') {
+    const { createWsStorage } = require('./ws');
+    storageInstance = createWsStorage({
+      baseUrl: process.env.REACT_APP_BACKEND_URL || 'http://127.0.0.1:4001',
+      wsUrl: process.env.REACT_APP_BACKEND_WS || 'ws://127.0.0.1:4001/ws',
+    });
+  } else {
+    const { createMemoryStorage } = require('./memory');
+    storageInstance = createMemoryStorage();
+  }
+  return storageInstance;
+}
+
+export function getStorageMode() {
+  return process.env.REACT_APP_STORAGE || 'firebase';
+}
+
+export {
+  initializeApp,
+  getAuth, signInAnonymously, onAuthStateChanged, signInWithCustomToken,
+  getFirestore, doc, setDoc, getDoc, getDocs, addDoc, collection,
+  onSnapshot, updateDoc, deleteDoc, query, orderBy, limit, writeBatch,
+};