Phase 3.4: client_app self-managed auth cutover

Rips firebase_auth; auth talks directly to the new backend endpoints.
Anonymous-first + phone OTP work end-to-end; Google/Apple SDKs are kept
but buttons are hidden behind ENABLE_SOCIAL_AUTH until backend OAuth
credentials are provisioned.

Smoke-tested against the backend via curl:
- anonymous → PATCH display_name → /me
- OTP request (read stub code from backend log) → verify with
  anonymous_customer_id → same customer row preserved, display_name
  preserved, phone added → upgrade confirmed
- refresh rotation + logout → post-logout refresh correctly fails
  REFRESH_INVALID
- Debug APK builds clean

- pubspec: drop firebase_auth; add flutter_secure_storage
- core/auth/auth_bridge.dart: shared mutable state (access token +
  refresh callback + in-flight de-dup) — keepAlive provider
- core/auth/token_storage.dart: flutter_secure_storage wrapper
  (customer_refresh_token key)
- core/auth/social_auth_enabled.dart: const flag from
  --dart-define=ENABLE_SOCIAL_AUTH (default false)
- core/auth/auth_notifier.dart: bootstrap via stored refresh; anonymous
  via /api/shared/auth/anonymous + PATCH display_name; phone OTP via
  /api/client/auth/*; Google + Apple wired (passes anonymous_customer_id
  for upgrade); anonymity config check for ForceRegister state; granular
  error-code mapping
- core/api/api_client.dart: Bearer from bridge + postRaw(skipAuth) for
  auth endpoints + single-retry 401 refresh
- core/chat/chat_notifier.dart + core/pairing/pairing_notifier.dart: WS
  auth frame reads bridge.accessToken
- features/auth/screens/otp_screen.dart: verificationId → otpRequestId
- features/auth/screens/register_screen.dart + force_register_screen.dart:
  Google/Apple buttons gated behind kSocialAuthEnabled; force_register
  drops obsolete linkAccount() (upgrade happens server-side now via
  anonymous_customer_id)
- client_app/CLAUDE.md: Auth section rewritten (was stale on Firebase)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

client_app/lib/core/pairing/pairing_notifier.dart

@@ -1,11 +1,11 @@
 import 'dart:async';
 import 'dart:convert';
 import 'package:dio/dio.dart';
-import 'package:firebase_auth/firebase_auth.dart';
 import 'package:riverpod_annotation/riverpod_annotation.dart';
 import 'package:web_socket_channel/web_socket_channel.dart';
 import '../api/api_client.dart';
 import '../api/api_client_provider.dart';
+import '../auth/auth_bridge.dart';
 import '../constants.dart';
 
 part 'pairing_notifier.g.dart';
@@ -114,10 +114,9 @@ class Pairing extends _$Pairing {
 
   Future<void> _connectWebSocket() async {
     _closeWebSocket();
-    final user = FirebaseAuth.instance.currentUser;
-    if (user == null) return;
+    final token = ref.read(authBridgeProvider).accessToken;
+    if (token == null) return;
 
-    final token = await user.getIdToken();
     final wsUrl = ApiClient.baseUrl
         .replaceFirst('https://', 'wss://')
         .replaceFirst('http://', 'ws://');

client_app/lib/core/pairing/pairing_notifier.g.dart

@@ -6,7 +6,7 @@ part of 'pairing_notifier.dart';
 // RiverpodGenerator
 // **************************************************************************
 
-String _$pairingHash() => r'93049804c1d55a0195a56b97d6e7f34fe6ab8086';
+String _$pairingHash() => r'a283e74d7cb4244bac74a950205c91d4b2cf3e9a';
 
 /// See also [Pairing].
 @ProviderFor(Pairing)