js: migrate Firebase from v7 (namespaced) to v11 (modular SDK)

[jpcottin]

Why

The js/ web demo pins firebase 7.19.0 (the last namespaced/legacy v7 release). Firebase v8 deprecated the namespaced API; v9+ ships the tree-shakable modular SDK and v10 dropped the v7 compat shim entirely. Staying on v7 blocks any other dep bump that transitively pulls a newer Firebase, and v7 itself has been EOL for years.

What

Single commit. Migrates js/src/components/login_firebase.js from the v7 namespaced API:

import * as firebase from "firebase/app";
import "firebase/auth";
firebase.initializeApp(config);
firebase.auth().onAuthStateChanged(...)
firebase.auth().signInWithPopup(new firebase.auth.GoogleAuthProvider())

to the v11 modular API:

import { initializeApp } from "firebase/app";
import { getAuth, GoogleAuthProvider, signInWithPopup, onAuthStateChanged } from "firebase/auth";
const app = initializeApp(config);
const auth = getAuth(app);
onAuthStateChanged(auth, ...)
signInWithPopup(auth, new GoogleAuthProvider())

package.json bumps firebase: "^7.19.0" → "^11.10.0". @react-firebase/auth (deprecated, only used as a v7 helper) is removed — the modular SDK does everything it did and more.

Test plan

Verified end-to-end against a Firebase project I own (not the dead android-emulator-webrtc-demo referenced in the in-repo config — see follow-up note below):

• npm install && npm start builds clean on react-scripts 5
• App boots, no v11 init errors in console
• Google sign-in popup opens, returns a user
• onAuthStateChanged fires, getIdToken() returns a JWT
• Token pushed to TokenProviderService.setToken as Bearer <jwt>
• App transitions to authorized state, renders EmulatorScreen
• Envoy (per js/develop/envoy.yaml) validates the Firebase JWT against the issuer/audience, proxies gRPC-Web to the emulator container
• WebRTC video stream from emulator container renders in the browser

Follow-up (separate PR)

The bundled js/firebase_config.json still points at android-emulator-webrtc-demo, which nobody currently has access to. That project has been broken since at least commit 5197542 ("Envoy not recognizing Firebase tokens"). A follow-up PR will restructure the demo to BYO-Firebase: ship a firebase_config.example.json template, fix the (currently no-op) substitution bug in config_gen.py, parameterize the envoy JWT issuer/audience, and document the setup in js/README.md. That's a docs/structural change, kept out of this PR to keep the SDK migration reviewable on its own.