M2 (C): storage barrel re-export, App.js imports swapped

- src/storage/index.js: re-exports Firebase SDK
- App.js: imports from ./storage (was firebase/* direct)
- STORAGE=firebase = identical behavior
- dev server compiles clean

Safe refactor proof. Next: per-call-site path-based rewrite for ws adapter.

src/storage/index.js

@@ -0,0 +1,13 @@
+// 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.
+
+export { initializeApp } from 'firebase/app';
+export {
+  getAuth, signInAnonymously, onAuthStateChanged, signInWithCustomToken,
+} from 'firebase/auth';
+export {
+  getFirestore, doc, setDoc, getDoc, getDocs, addDoc, collection,
+  onSnapshot, updateDoc, deleteDoc, query, orderBy, limit, writeBatch,
+} from 'firebase/firestore';